You just cant beat the taste of fresh garden pease!
I like them young and sweet (petit pois) so I pick them before they get too big. Its always a good idea to open 1 or 2 pods to see how their developing and sample the taste, this also gives you the chance to make sure they dont have magots too.
Somtimes you can get a pretty good idea by holding the pods up to the light to get an idea of the pease inside. Good Luck
A good rule of thumb (no pun intended) is to have the pods be equal in diameter to just between the size of your little and ring finger. Thunderbird's ideas will work as well...