Definitely get a fibrewrap bottle rather than a steel bottle, as the weight difference is huge.
As motocross boy has already said, 68ci (1.1litre) bottles seem to be the most popular choice as it provides enough air for around 1000+ shots and doesn't weigh a huge amount.
Depending on your position when playing, a 45ci will most likely be to small. The last thing you want when you are playing, is to run out of air.