I have a Weedeater 18cc engine that I am converting to airplane engine and I would like to know what size plane an engine like this would work in if any. Also if there is anyone out there who knows how to compare gas to nitro. Example, when they list the plane as a 120 size what size gas engine would work on a plane like this

Anything under 80" is generally borderline between too heavy and not enough wing area for a heavier gasser.

I can't see an 18cc engine flying much.. really depends on the final weight of the engine. That engine might fly a 65" - 72" high wing style plane.