As someone who compiles these ships into mods. I won't use anything above 512x512 - I'll always scale 1024 textures down. It's never yet made any difference. Although scaling from 512 to 256 CAN make a difference, especially on models which are high detail, but have a low number of textures (P81's for example).
I always convert to 8-bit, the difference is absolutely tiny, and the amount of space saved is around 70%.
Many P81 Models used up 20-30MB originally (on my HDD), but I've got that down to around 2MB in many cases.
That's how I managed to squeeze a 300+ Model mod into 170MB.
Of course, Modellers always release their Models in the highes possible resolution, that's expected really. But it's usually not suitable for inclusion into a Mod, not unless you want it to go over 400MB.