This is sort of three questions, but intimately related. The ZPM pages on Stargate SG-1 Solutions and the Stargate Wikia have some information, but not all reliable of referenced.
The questions are:
We don't have a lot of information about ZPMs beyond what the show tells us:
Suffice to say that if humans knew how they had been constructed, we'd be making attempts to construct them ourselves.
We do know that they can't be recharged. They draw power from an artificial subspace region that they are tied to (presumably when created). Once that region has reached maximum entropy, the ZPM is dead.
There is no indication that the ZPMs had any significant difference in maximum charge. Presumably, one subspace pocket is substantially similar to all others, and contains the same potential energy. It may stand to reason that a smaller pocket would contain less potential energy, but there's no reason to suspect that the Ancients ever made a ZPM with less than their maximum potential output. It's possible that while they were being actively developed and produced, the Ancients had ZPMs with smaller maximums, but the ZPM seems to have been standardized in the final known form long before the Alterans began doing all the things they're known for.
