cpt crunch pretty much knocked it out of the park for the first part of your question. First number is volume in cubic inches, second is max pressure you can fill to, etc. etc.
As for suggestions. First, spend the money on a carbon fiber wrapped, 4500psi tank. It's worth it. Steel 3000psi tank are a pain in the butt. They're big time heavy, and give a relatively low shot count, unless you get a big one, which will be even heavier and more annoying. Yeah, maybe you're a tough guy and "weight doesn't matter" because you've got some muscles or whatever, but save yourself the irritation of having a tank that is probably going to be heavier than the rest of your setup combined.
Brand wise, it seems that there are two companies that are far and away on top of the heap nowadays. First, is Crossfire. I'm a Crossfire guy. I've had one of their HP fixed outputs for a lot of years now and aside from need a new fill nipple every few years or the occasional tank o-ring it has been an outstanding piece of equipment. Second is Ninja. The field I work at rents Ninja tanks, I have been pretty impressed by them as well, their regs are small, simple, rebuild-able, and can take a fair amount of abuse with no ill effects. I feel that they're an all around good option, their popularity is well deserved.
Size wise, I've never really bought in to the goofy sizes they're coming out with these days, 48's and 50's and whatever other goofy numbers they're selling.
68ci is a good go to general purpose size. It'll get a fair to good amount of shots on any style and type of gun. Today's more efficient markers will get almost a whole case from a single full 68/4500 fill. Even if you don't have a super efficient gun it'll still give you at least half a case on just about anything.
Personally, I've been using a 45ci tank now for the last few years, the small size and light weight is definitely very nice. I can get plenty of shots with a full fill on my poppet guns, and she works nicely with my Cockers and Snipers as well.
You might also consider one of the "stubby" designs out there, I just got a 72ci 4500psi stubby tank from a friend, it's short and fat, so length wise it's about the same as my 45, maybe only a half inch or so longer from butt to the tip of the reg and would give you almost a 2/3's increase in shot count.
At the end of the day though, which size tank is right for you will come down to personal preference.