I posted this on the Aerocycle rider thread, seems to apply here as well:
Everyone that says that vintage bicycles with 26" wheels and an 18" seat tube were made only for children are simply wrong, plain and simple. The bike shop down the street from my office is chock full of modern bikes that frame size or smaller and all are primarily intended for adults. Many of these manufacturers made tall versions of balloon tire models, typically more during the prewar era though. I see tons of vintage photos right here on the Cabe of adults and their balloon tire bicycles. People were shorter in the 30s as well.
Also, I think as the lightweights gained popularity among adults the balloon tire bikes became more youth oriented. Probably not a ton of college kids in the 50s riding Phantoms and the like but I could be wrong.
As has been pointed out, there were longer seat posts of different configurations as well as many handlebar choices to accommodate riders of all sizes. If you want your bike to look like an actual vintage rider then adjust the cockpit accordingly. If you want your bike to look just like the catalog shot then it probably won't be too comfy to ride as a full grown adult.
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