I went for 50L. It depends what you are going to use it for and whether you want the thing running full time, which will happen if you buy one with a small capacity tank (as Blocky says) and it heavily.
I did quite a bit of research and concluded that noise was also a big factor factor for me as I have decent neighbours who I wouldn't want to upset. The stack 'em high sell 'em cheap stuff have high db ratings of 90db plus (some advertisers don't show the db ratings and don't even repsond when you ask the question!!) and you'd want ear defenders on if you were running that in a garage. Their are really expensive quiet brands such as Bambi that run at 49db (conversation level) but they are really intended for dental surgery applications and although they be fine for a stapler or inflating tyres they wouldn't powere a spray gun or a sander. This is because they employ fridge compressor technology In the end I went for a sensible compromise of 72db which should be ok, the reckoning by experts is that anything over 80db is unpleasant. If mine proves too noisy then I will build a soundproofed enclosure around it with a cooling fan built in. Belt driven types are quieter than direct drive versions.
Thanks for the advice regarding hoses and fittings too guys