For relatively recent homes (last 40 years) I'd say the secondary bedrooms tend not to be roomy, with exceptions of course. The builders and buyers try to keep marginal square footage down, an easy target for this being the other BRs. I'd also guess that more of a homes marginal square footage went into baths, kitchens and living areas (e.g., two vs one).
Having said that, I'd say the above 120SF assumption is likely close. A 10x10 would be small, a 12x12 fairly generous, so 11x11 or 11x12 more typical, the average of those = (121 + 132) / 2 = 126SF. Again, guesses. Online listings now often show room dimensions, so check out Zillow or Realtor.com and see the actuals in some areas you understand.
Remember that BRs contain in most cases a closet and often an entry "hall" that is the depth of the closet or the length of a entry door, so thse gets subtracted from the overall dimension when looking at useable train layout and surrounding space.
If you want to double your fun, design a HO and an N layout, which of course could be completely different, plus illustrated the tradeoffs a modeller could consider.
