sized properly. Buy one that is too big and you have a large delay when the system is activated. It is hard finding accumulators that are less than 50cc's.
What I did in the mean time was to attach a long hose to my pressure gauge so when I was tunning the system I could see the gauge while I was driving. A positive side effect was that the air trapped in the long hose was essentially a rudementry accumulator. As long as the tube does not get filled up with water over time it will be good for a temoporary solution for now.