you really should have one, like anthem said, it is a big chance, but you don't have to have it
how big depends on the fish, but even a 20 will work for mosy fish and would be far better than nothign(they will only be in it a couple of weeks)
i would qt new additions at least 3 weeks, or probably even 4, before adding ot may main tank, but other than that it pretty much depends on why it is quarentined
and i really do reccommend quarentining every fish before addin it o the main tank, it is much easier to tereat a problem there, than to try and treat a whole tank once you do have a problem
all it takes is one time, and we wish we had done it first
HTH and good luck