Thee really isn't a size limit, however the plane must resemble a plane that has or could fly in an aerobatic box. We currently have a 60 size electric Extra in our region that flies Sportsman and he does well with it. He does fight wind harder than those flying 50cc planes and heavier, but he does well.

One thing to remember in IMAC, light weight is not always good. Weight allows better penetration in high winds. In our region and I suspect most others, we will not even consider not flying until the wind hits around 20mph to 25mph and then we'll hold a vote. A 10lb plane is going to be hard as heck to fly in that wind.

If you are just starting out, I would go and start in Basic. It's a bring and fly what you have type of class, any plane can be used. Don't let the term Basic fool you. It's a lot harder than it looks. Once you get into Sportsman and above, you get to fly unknowns and if you are not familiar with Aresti, then that can bite you as typically the unknowns can change the outcome of a contest quickly. The unknowns are handed out the night before you fly them, you do not get to practice them at all, excet with a stick plane.

Basic does not do unknowns.

Remember this, you must always fly a perfectly wind corrected line at all times (horizontal and vertical), the wings must remain level at all times. Do your first contest in basic, then go to sportsman if you do not feel challenged enough

I know I went past the original question, but that question lead me to think you are just starting, if not then I apologize