That covers a lot of territory and considerable variations in size regardless of wheel arrangement. A Cotton Belt 4-4-2 was a real lightweight and only had 70" drivers. An 80" drivered Pennsy E6, on the other hand, was more powerful than a number of Pacifics. Even as far as 4-8-4’s go, they were all over the place in size varying from 361,000 lbs (engine only) on the TP&W to 510,000 lbs (engine only) on the AT&SF with drivers ranging in size from 69" on the TP&W to 80" on the drivers of the UP, SP, ATSF, and ACL. GN’s O-8 Mikados were bigger than some 4-8-4’s, so it’s all relative.
I think what you’re getting at is more of a concentration on middle-sized engines. For instance, an NC&STL J-3 or Cotton Belt L-1 sized 4-8-4 would be more layout friendly than a GS-4, an NYC S-1 or a Santa Fe 3776/2900 class. While both a USRA Light and Heavy 2-8-2 are moderate size engines, the light’s a bit more in keeping with the relatively short trains we can run as is the light Pacific in comparison with the Heavy.
What would be nice (although I ain’t holding my breath until it happens) would be some lighter Pacifics than the USRA light (say something along the lines of a Harriman light, a Santa Fe 1226 class or even an NKP K-1). A Harriman Mikado would be a nice contrast to the USRA engines and could be used alongside USRA engines as examples of earlier Mikado purchases. </