Regarding walkmeshes, the fundamental concept is that they cannot overlap from tile to tile. The dimensions are imposed to be 10 meters in each horizontal direction (x and y). To my knowledge, there isn't any limit on the depth or height (z axis).
Regarding geometry, there aren't any limits, except that any geometry below z=-10 meters will not cast any shadow. Tile geometries can overlap (the ttr01 inn does that, for example). One is of course advised to always keep a reasonable number of polys.
About the height of a raised level, Bioware picked 5 meters in their tilesets, but community builders have used different levels in their creations : DOA's rural/city has 4 meters, for example.
I hope this helps.
Modifié par Nissa_Red, 27 février 2013 - 11:00 .