From my experience for ground vehicle is enough GOOD 256x256 texture.

Surely more= the vehicle looks better,but the demand for memory increases very much.And imho it does not make sense.
Thealx and Ignarik helicopters texture looks fantastic.
It was a shame to decrease and 1024x1024 let him be.

All other textures larger than 1024x1024 It does not make sense, because in the game you can not see difference.
My 2 cents.Maybe you have other and we determine some standards.