I'm new too, but I'll try to be helpful with the knowledge I have. Anyone, feel free to correct me if I'm wrong.
FX armor is generally slightly larger, specifically the helmet. As it was fan sculpted, it might not be 100% accurate, but from an overall perspective it suits the "that's definitely a stormtrooper" needs if not more. I've read that RT might even be slightly bigger than FX.
I believe FX lite is sold without the helmet (body armor only) where as the Full FX has everything including the helmet.
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In terms of helmets, they usually come as one size fits all, and it's up to you to install foam padding or an inner helmet harness (from a hardhat or military like helmet) so that it fits your head.
There's a variety of fan kits available, but you can also head to most electronics stores and buy small fans made to cool down PC computers or electronic parts, where you can just easily wire them to a battery or switch. I've done some research and the prices have a huge range for pre-made kits, so surf around first.
Green flat eye lenses are usually used for stunt helmets, dark grey bubble lenses for hero based helmets. Depends on what you like. A lot of the armor makers make complete kits that come with everything (decals, etc..) and others require you to get them separately, or some choose to upgrade. For example, some come with black cheek decals and others blue (to match the film) so that's up to you.
On top of an armor kits, assuming it doesn't come with everything, you'll need industrial velcro for closing openings, webbing to suspend / hold the armor together, rivets, snaps, screws for the helmet, some foam for shims, and a few other odds and ends.
It's likely you'll need a belt to mount the belt parts / thermal detonator on. A holster if your kit doesn't come with one, a black under suit, black gloves (rubber or Nomex), neck seal if you want one, and approved boots. I think there's only 2-3 companies making boots that are "approved" for use, like Caboots, etc..