for fighters and bombers, the approx poly count is around 1-2k.
for small freighters/gbs you're looking at about 2-3k ish.
cruisers 3-5k, same for med/large transports
BCs about 5-7k at best, battleships can be around 7-12k.
there again, there are exceptions where certain ships have more or less polys. like i said, its a general "guideline", doesnt mean you HAVE to be exactly in that range. it can be a few polys up/down etc, as long as its not too extreme- for example, a VHF with 8k polys wont fly.
for texturing for discovery, you should be sticking to using vanilla textures. sizes can by anything upto 1024/1024, though there are some (very very rare) textures that are 2048/2048. avoid going bigger than 1024, if you decide to make custom textures, and another point, they have to be seamless, so that they can be used multiple times (like all vanilla textures etc that are used in disco, they are all seamless).
even for your own mod, these are the "guidelines". remember, many people who play fl dont necessarily have top end machines, so loads of ships with high poly counts or huge textures will cause epic amounts of lag. that said, you can achieve alot of top notch work with a 64/64 tile just as well as you can with a 128/128 or a 512/512, etc, so dont forget that.
texturing should be done properly. half arsed jobs which end up having unmapped faces on the ship, distortions, warped faces, etc should be dealt with accordingly as well.
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that said, its nearing 11pm. i need to get to bed, toodlez.