hi team im wanting to get into gas planes whats the best size to learn off

As long as you are aware of the weight differences (20cc vs. 20cc, 30cc vs. 30cc and 20cc vs. 30cc) I don't know how you could go wrong with either? If I had to pick, the 30cc's might not be quite as picky on their needle settings? To me though, the more compact air frames used with the 20's make them friendlier from a handling (hanger rash) perspective, and are easier to transport. If you're as cheap as I am, it shouldn't be hard to see the 20's are a little less money as well - engines AND airframes...

I've a couple of DLE20's and am at least as cheap as ahicks couldn't agree with his post more!

IMO use the 20 cc engine when the all up weight of the plane is 12 lbs or less and the 30 cc if overall weight is above 12 lbs.

+1 for what Indiomike stated. I have both DLE 20 and 30 and basically agree but you may also want to consider the drag factor
of the airframe your going to use the engine on; monoplane vs biplane for example.