Half litres are fine for getting you to nang town and back. Make sure you use quality bulbs though (not MOSA) as the residue from the shitty brands tends to build up pretty quickly in the smaller machines and nothing sucks more than having to clean your canister mid-doof.
I only feel comfortable squeezing two bulbs into a half litre canister. Which is enough, as two in the machine and two in a balloon is really about as many bulbs as one can do at once.
I'd have to disagree with the MOSA comment. I have a 0.5L canister and I only manage to just fit 1 nang in (putting a second in results in most of the first rushing out the safety valve.