Kinda depends on what marker you are shooting, where you intend on playing on the field, how long your arms are and how often you can be bothered to fill your tank. If you post this stuff up, i'm sure someone will be able to advise you
All things aside, the most popular size is probably a 1.1 litre fibre wrap system from the likes of crossfire or pure energy.
Edit:
Stakos are renowned for being the lightest air system on the market, however there have been instances of accidents involving air tanks, and some people argue that more often than not, stako bottles are involved. I would do a lot of reading around stakos before i decided to buy one.