any one know what prop size to use on a rossi 40 for 3D flying , 5 to 6 pound profile plane ........

APC 12 1/4 X 3 3/4

Yepp, or APC 12-4 (which is my favourite)

APC 12 1/4 X 3 3/4

and put the plane on a diet if it weighs 6 lbs - that is too heavy! Sub 5 lbs is your goal.

Yea, I missed the 5-6 pound part. It'll never hover at that weight.

You need at least enough power(thrust) to weight ratio to pull out of the hover at full throttle.

Look to cut weight with a micro servo for throttle, and featherlite foam wheels for the mains. You can eliminate wheel collars where possible. If you weigh all of the excess axle metal that you cut off, you would be surprised at the total weight. Dubro makes 1" foam wheels for electrics that make great tail wheels.

My Burrito and Mojo weigh in at just slightly over 4 lbs. I use a strong 46 with good results. Sounds like you need a 60 2-stroke.

I thought you were kidding in the 3d forum. Apparently you are serious so here is a straight answer. A Rossi 40 likes to scream - 16,000, 18,000 rpm. 3d is about flying at 1/3 throttle with enough reserve to pull the plane straight up, ideally accelerating. Thrust to weight of 1.5 is common. Lots demand more like 2. I fly 3d a lot. My favorite plane are 5.5 to 6 lb Mojo 60's. I fly them with 4 stroke (Saito 100) engines. I sometimes fly 4 lb 3d planes with 2-stroke engines - .46 to .50. I like 4" pitch on the smaller planes. 12" to 13" diameter. 60 size are a lot easier to fly.

A Rossi 40 on a 3d plane is like a Porshe engine on a John Deere tractor. You could make it go but why? It is totally wrong for 3d as I know it. I like Porshe and I like John Deere, but not on the same machine.