Deciding on size depends on a couple of things, firstly how big you are. You need a tank that you can handle without it getting in the way. Secondly how much air you may need. If you're a big guy there is no point going for a peanut tank, and if you're a small guy going for a 1.5 ltr tank. If you play day long games then the annoyance of filling up a small tank will make you crazy, but if you only play quick 20 min scenario games then you will be happier with the reduced weight. As zondo said the ninja tanks are very good, but not all the sizes and colours are available here, but the range is improving. The best thing you can do is try a few tanks on your marker, to test the size I'm sure your local paintball store will help.