When producing a scale model can you have scale models that are large or small? Or do all fine scale models have to be large? Thanks for your help.

Scale can come in any size you desire it's just that the large ones seem to fly better but it depends on the design. WWII fighters should be larger to apply all the details which add weight.
WWI aircraft as well as Golden Age aircraft have designs that work as well as trainers. Example the Curtiss Robin has dimensions and design that would flatter a 20th/21st century RC trainer. A bit of dihedral 1 or 2 degrees, not really enough to notice, would probably make Lindbergh's Spirit of St. Louis flyable. Either that or aileron differential. The scale Piper PA18A is a duster that has a lot of scale add ons such as dust spreader, spray booms and wind driven pump. It too should have 1 or 2 degrees of dihedral but not enough to notice.

Simply put, Yes! and

No.

The thing that makes the difference is what level of detail you can reproduce. For me, the bigger models are easier because the details are bigger. I have seen some very detailed cockpits, but they were on pretty big airplanes. In smaller scale planes, sometimes details can be "implied" (i.e., hinge lines, trim tabs, and the like, drawn with a pen) but still considered finely detailed models. Where as on big aircraft, these are usually actual working pieces--I have even seen working trim tabs. Most of it depends on your particular level of skill and patience. I have a little (16" ws) P-40 with working ailerons, flaps, elevator and rudder, but it way to little to fly with rc. I know I haven't been much help, but there really are so many variables, it is a difficult question to answer.