What size tank would be most comfortable for them? I don't want to keep the poor thing in a tiny tank, I want it to be comfortable.
Can spiders of the same specie share a glass tank?
What sort of spider would you recommend getting for someone who is new to this? Something that's low maintenance as don't die too easily.
Tank size/shape really depends on which species you go for. Tarantulas can be terrestrial, arboreal (live in trees) or fossorial (diggers), so you'll need to ensure that you have the right type of tank as well as the correct set-up inside. Tarantulas don't really need much room to roam around in as they're pretty sedentary apart from when hunting, so you don't need a massive tank. They tend to hide in the corner if you do. For example, an adult Chile Rose (
Grammostola rosea) can quite happily live in a 12"x8" fish tank.
Most species, when put in a communal tank would merely attack/kill one another immediately, so you'd end up with 1 very fat spider. Only a few species are recognised as living communally, mostly species of the
Poecilotheria genus. However, species should not be mixed and it's a good idea to have spiderlings living communally out of the same egg sac. This genus also consists of fast, arboreal spiders with a nasty bite and certainly wouldn't be recommended as a beginner's spider. There is the chance that they just wouldn't get on as a group and carnage would result, so you have to take your chance really. I've had 2 communal groups of spiders, both of which were progeny from my spiders. However, whilst one set did great, the others ate one another at about 2" legspan.
In terms of a beginner spider, you can't really go wrong with a Chile Rose (see link above). They're docile, very easy to look after (can be left when you go on holiday once they're adult) and very hardy. You don't even need a heat mat for them. They grow extremely slowly, so I'd recommend that you got a young adult (get it from someone who knows what they're talking about - you don't want a male). They also live for 30 odd years, so make sure that you really want one before you buy. For care requirements, Google "Grammostola Rosea care sheet". You'll find a lot of contradictions between different caresheets, but just take the main points. They like dry substrate, with a little water bowl (that can't be fallen into), a piece of cork bark to climb on and some substrate to dig in. Don't buy a tall tank because they climb sometimes Tarantulas can rupture their abdomens by falling.
Anyway, hope that helps

There are tonnes of forums you can turn to for advice. Arachnoboards is a good example. Word of warning though: ignore what they Americans say about tank size. They are completely obsessed with giving their Spiders tanks the size of Jupiter and it's not needed.