I would personally opt for the smallest guage possible for IV use but I may well be biased due to having appalling veins in the first place. Even before I started shooting up it'd take docs ages to hit up a vein long enough to take a few vials of blood for testing - several years after quitting pretty much all IV use and it takes literally weeks with multiple appointments for non-emergency venal access and, pretty much always, ultrasound and multi-inch needles for deep vein access for emergency situations.
Take very good care of your veins. They never, ever, get better than they are right now at this very moment.
Fubz is spot on with the citric call though. Hey, Zeus, fuck! does that stuff ruin veins severely and rapidly. Vit C powder is your friend and I'd personally go with nothing larger than standard 1ml syringes for IV use - or 0.5ml size if you don't mind paying for pins. Can't recall the guage-size offhand but these things are standard so easy to acquire at any pharmacy and/or needle exchange.