Can anyone tell me the rule of thumb for picking the right size spinner?

It depends on the plane itself. You match the spinner to the cowl or you just put on what looks the best to you. I just used a big odd shaped spinner because it was the heaviest spinner I had, it helped locate the CG. I also miss measured the plug when I was making a glass cowl so when I cut back the cowl it fit better then any other I had on hand.

+1 I go with what matches the cowl. I really don't like the small spinners on the larger cowls. Of course, in some cases I use what I got

Typically you use a spinner that is about 1/5 of the prop diameter (rule of thumb). Of course if the cowl is designed for a spinner, the choice has already been made for you.

From an engineering point of view, for an IC engine you should use a spinner that ends just below the lowest cooling fin. This gives you the most effective cooling when using an exposed engine. For cowls that have intakes it should end right at the edge of the opening again for best cooling. In reality, unless you have cooling issues, or need max efficiency, use what looks best to you.

It depends on the aircrafts cowling as stated

I don't think there is a rule of thumb for this one...As stated you pick one that fits the cowl and the prop etc.make sure that it is not a sloppy fit on the prop shaft, if it is find a little shim to take up the slack..Now If you have a cowl there will usually be a ring around the prop opening..That will be the size of your spinner back plate. ( Usually ) If you dont have a cowl you can just pick a spinner that you think looks about rite...Check out a few of your buddy planes and go by that ..Sort of like bra size's.. OH I like that one I tried to use a smiley but I don't know how , maybe somebody could teach me...Fitting to your prop shaft is important and not always as easy as it sounds..If you need to drill it you need to clamp it because the bit will try and grab and your hole will end up off centre..So useing a reamer might be the better tool for the job... Fitting the opening around the prop often needs to be done also...Just make sure it is not touching anywhere. and take the same amount from both sides to try and keep the balance ...

I think an engineer came on once and did some big math equation showing what size an uncowled engine should have on it for racers. Been a while but that could have been the rule of thumb. If not then I haven't heard of it before. Some people do use a spinner with cooling in thought or air penatration but the spinner is just used so the electric starter has a nice place to grab onto when starting the engine and they really give a cowled engine that clean look. You can also use spinner nuts instead of a real spinner for a clean look. I'm using perhaps one of the ugliest spinners in the world {unless it's a warbird} just so I had enough weight to get a CG and had something for the starter to hang onto. Because I can't see to operate a ruler correctly I had to cut the cowl I made and it made the spinner look even dumber but I will make a new cowl when I get the time, maybe even mold in this big spinner? anyway, different types of spinners and nuts shown.

Yikes I gotta agree with ya Gene that is a wee bit a on the different side however your cream can door ornament completely makes up for it...That is really nice...The Swoose looks great anyway.. Have you done the maiden yet ? Did you get a chance to see John? I dont know how close you guys are... I know he is home now...Is he in Vegas too? Really that spinner is not bad, just different..I can see where it would belong on a war bird....With the bent wings on the swoose it kinda belongs...

No on the maiden but any time. Sense I took it out and discovered the bad battery pack I have made up a new one. Battery Plus had some AA batteries with the same chemistry as Eneloop, $15.00 for four 2000mAh. Then a storm came in and and we have had wind and rain for several days. The dry lake bed I fly from is flooded again! There was a rodeo in town for several weeks and South Point used our kinda secret field as the over flow RV lot with horse stalls. The rodeo was over Sunday. We went to the South Point yesterday to the theater to see the Hobbit, I drove by our secret field and all the RVs are now gone but they haven't removed the horse stalls yet. Not a cowboy hat in site at the casino so I hope to see the stalls gone this week? If so and the weather changes I will get in the maiden then get the floats on the Swoose and go out to the not so dry lake bed and try the water maiden. I still have a set of waders from my duck hunting days so the water maiden could be a lot of fun.
I didn't know John was in town or I would have went by. He hasn't said but he could have been in the hospital down the street from me. If he comes back for his tests and check ups I would take him over to the cheap prime rib place for dinner if he tells me in advance when he will be here. John probably knows the strip better then I do but the prime rib dinner at Ellis Island isn't bad for the price. No show girls but some good looking tourists. Our old hobby shop has also sold, not near as good as it once was and it has moved locations so I would show him where it moved to.
OH, forgot, that big spinner has a solid 1/4 inch plate in the back instead of the normal little spinner plate like most of them. Nick Ziroli had to add a pound of lead in the nose of his proto type to get the CG. I knew going into the build the plane would be tail heavy so everything is mounted as far forward as it could go, I used the YS 1.10 for power, twice the size of the proto types engine and I still needed almost 2 ounces of lead mounted onto the motor mounts.
I miss measured when making the foam plug for the cowl and you can see I had to cut about three inches off of it. If the little plane is fun to fly I will make a new cowl for it. If it's a slug then what you see is how it will always be. I really tried to keep the aft section as light as I could. Plane #2 is way lighter then this one.

That motor looks huge on there...It would fly my 1/4 Cub...I dont think its going to be a slug unless its a very fast slug...Sort of a super slug...Its a good thing you have long legs on it..What an ideal float location where your recovery boat is a set of waders...Just dont get too far out...How cold is it there this time of year ? We got 16" of snow sunday..Wet snow...I can float fly too...

When the dry lake bed gets water and floods it's only a few inches deep. It's 7 sq. miles of flat. It's 27 miles from my home. We have a kinda secret field 7 miles from my house but it was used the last few weeks as an RV horse stall over flow for the big rodeo that was in town. Surprise!! Vegas is big on rodeos. The rodeo was over Sunday and all the RVs are gone from our secret field but the horse stalls were still there Monday. When they are gone and these storms move out of the area I will get in the maiden. Depending on the water level on the lake bed I may or may not get in the water maiden.
To clear the prop I had to bend longer legs for the floats, the plane looks better with shorter legs on the floats. I had to put larger tires on the land gear too. I took a nip out of the prop just on engine run up.
Anyway, it's been ready to go for a while now. It looks fast but it's just a flat bottom wing like a trainer. Prop selection will determine speed and I strive for slower planes with a lot of thrust. These two planes were built just for two old guys to be sitting on a dock and relaxing on a nice summer day. They were both supposed to be electric powered but my temper got the better of me when trying to figure out the motor and ESC. After I set mine up with the YS we finally figured out the motor and ESC size. #2 is much lighter but he hasn't gotten any farther with it sense I gave him his plane. Once mine flies he will get going on his.