How do you all select 4 stroke size for a vintage airframe, or visa versa? Older kits that have no 4 stroke recommendations.

I take the recommended 2 stroke size and multiply by 1.5 as a starting point. Seems to work well for the average sport flyer. If it's a slow lightweight high drag trainer airframe I may scale down the engine size from 1.5x. If it's a well built slippery airframe I may scale up the engine size several displacements. Same with 3D planes, I may scale up a size or two from 1.5x.

For 3d it's mostly about thrust (prop diameter)
For fast planes it's mostly about pitch speed (prop pitch)
For sport planes it's all about having a good balance between thrust and speed.
For trainers its all about not having too much thrust or speed, unless you don't mind seeing a flimsy trainer turn into a balsa cloud.