Plane: Low wing trainer, 450-500sq-in, 4.5-5lbs (~22oz/sq-in), tricycle, 2.5" wheels
engine: old OS 25FSR (hopefully still stronger than LA).
Possible props 9-5, 9-6, 10-4.

Wing loading is a bit high, so it needs speeds for takeoff (high pitch). However, the engine is small so in order to get some static thrust
to move in the grass, you'd expect a larger prop with lower pitch.

What's the best compromise here for easy takeoff from grass?

I'm a big fan of APC props for they seem the most efficient, I'd get one of each size and see what works best for you. All the opinions you'll get on a specific prop from people here might not be right for your set-up so spend the little bit to buy an assortment. Eventually you'll use them all if you stick around the hobby for very long.

Have a look at the wheels as well!
A bigger set of wheels will allow a bigger diameter prop and they will also roll easier on the grass.

I agree. Big, light weight, tires should help (at least 3" diameter light foam), They may look kind of funny, but should help.

And generally you'll want larger diameter/ smaller pitch prop to shorten the take off roll. But it may well depend on the engine. Buy the 9X6 and the 10X4 at least, then try the 10X4 first.
Dennis-

Wow, 22oz/sq in! Are you sure you even want to try flying it?

I just took my 25FSR off a 3.4 lb plane with a 437 sq inch wing (17oz/sq foot). Nothing wrong with the engine, the plane is my testbed for new aquisitions. It's temporarily got a OS 32FSR. With the 25 FSR, I used a 9x5 Master Airscrew (MAS) prop. The plane is a taildragger with small (<2") wheels. Performace was great, no problem with takeoffs from grass. For yours, I think the 10x4 is the best prop to start with, but you might also try the 9x5. I think the 9x6 might not let the RPMs get high enough.

I've seen some discussions about the relative efficiency of MAS versus APC props. Last I recall, it was not clear that APC actually were more efficient. There were some comments that the narrow tips of the APC props let the engine develop a few more RPMs, leading people to think that they were more efficient, but that the actual thrust might be a bit lower. I think it boils down to personal preference. I use both. I like the look of the APC, but I think the flat tips on the MAS are a bit more durable, and not so sharp.

Your wheels should be ok, but if your field is rough, or the grass is long, do as has been suggested, go to a light 3" wheel. Be sure to give the plane plenty of time to get up to flying speed before trying to take off. If you can, adjust the attitude of the plane so it sits slightly nose DOWN. If you can't adjust the height of the landing gear, use a smaller nose wheel. This will help keep the plane on the ground until you actually give it up elevator. It will also help it stay down when you land it a little too fast.

How long is the grass? Is it mowed regularly.....or is it like ours where only two people mow and they're the ones with the busiest schedules? If it's like ours......I's say forget the wheels and use skis!!!!!!!!! All kidding aside, I'd go with the largest prop and work my way down.
May your head always be up in the clouds.
xs

22 oz/sq ft not 22OZ/sq in.
I now take off with an inferior plane, same wing loading and 2.75" light wheels. On our grass I cannot really train my maneuvering and do slaloms. I actually rolled over a ridge created by a mole. But I can just "floor it" and take off . I want my low wing trainer to handle better and take off easier. On grass, which handles better, a tricycle or a tail dragger (both versions are on the plans)?
BTW it looks like it will not tear up the sky with 4.5 lbs and a bearing 25.

[quote]ORIGINAL: Gold
I now take off with an inferior plane, same wing loading and 2.75" light wheels. On our grass I cannot really train my maneuvering and do slaloms. I actually rolled over a ridge created by a mole. But I can just "floor it" and take off . I want my low wing trainer to handle better and take off easier. On grass, which handles better, a tricycle or a tail dragger (both versions are on the plans)?
[/qoute]
Trikes are almost always easier to handle than tail draggers. Tail draggers are harder to keep going straight and can be easy to nose over, especially when there's alot of resistance on the wheels. Having said that, I still prefer tail draggers. I actually learned to fly with a tail dragger. The two planes I have flying right now are tail draggers. Only 4 of my 13 kits waiting to be built will be trikes. Taildraggers are lighter. I think they look cool when the tail raises up off the ground on takeoff, and while scooting along the ground during a touch and go. Since I don't know your flying skills, or preferences, I can't tell you which way to build it. However, if it was me, I'd go with the tail dragger choice.
You don't need to tear up the sky with your first low wing plane. Save it for one or two planes down the road, or once you are comfortable with the plane, pick up a 32 FSR or FX off eBay.