that depends on what kind of strain it is , the technique used to grow and the lights ofc .. if you're trying to keep them short then you should top/fim and LST.
are you gonna transplant/separate them ?
People 12/12 from seed, so yes. Roll a joint and start reading through other grows on this forum. You'll pick up a ton of tips.
Final plant size is determined by lots of factors, and no one will be able to accurately predict it without as much information about your grow as possible, including pics.