Poserworld may be the best shot for WWII uniforms. Civilian clothing from that era probably harder to find except a piece here and there. Most ‘vintage’ seems to go pre 1900, Victorian, Edwardian. I’ve seen a few ‘flapper’ era things, but again not easy to find.

Ah, I remember the Poser scale wars – good thing the UN finally sent in peacekeepers. I don’t know if even DAZ uses some kind of consistent scale between their figures, much less thinks in terms of real accurate biometric scale. It seems like you’d need them and probably SM to agree and maybe the independents would follow. It’s true that when you get into actual interaction with the figures, Poser and non-Poser content, scaling is going to be critical – as in the trumpet example. OTOH, the trumpet lying on a table or someone just holding it, some simple scaling might suffice. The same for furniture etc. If it looks like she ‘could’ sit in it, it may make a better scene than an empty one. Not everyone’s going to whip out the calipers and cry foul. Some type of photo mapping or use of images as mentioned can help too.

Unfortunately, Poser kind of evolved outside the mainstream of 3D and back then, there probably wasn’t as much standardization even between CAD programs etc. Nowadays, different companies are exchanging stuff, sending plans directly to fabrication etc. and it all has to match. It would be great if we could someday have that level of consistency. Of course, the occasional space probe does go missing because someone forgot to convert units …

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken