This free paper model of a Roman villa seems to be somewhere between 15mm and 20mm size. You could scale it on your printer, though to make it work in other sizes.
Warartisan is offering a free sample model of the Enterprise, one of the ships that fought on Lake Champlain in 1776. It's part of paper model ships to go with the free Away Boarders rules for refighting naval battles on the Great Lakes in the 17th and 18th centuries.
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At the Game Architect blog, you'll find a good looking paper model of a 15mm American Civil War wood and stone barns. The models are in black and white, so you'll have to apply your own coloring.