I don't think theres a thread for this but heres a brief rundown.
Small units - 1000<
Big/Special units 1000-2000
Buildings 2500<
Tileset objects - 200<
Big/Special objects - 500<
Tileset objects and very small and uncomplex units should use 256x256 textures. Special tileset objects, most units and buildings should use 512x512 textures.
Glest can happily exceed these requirements by several times, but the above values will keep glest chugging over on lower end integrated card machines.
For an example of a higher poly mod and bigger textures try running the Dwarves mod. For an example of very excessive over use of polies try the Malevolent Rising mod (some units have 10,000-15,000 polies!!).