To answer Mike's question, the outside dimensions of the rectangular battery are 30mm x 28mm x ~2.6mm. However, that's not the full story, as the part of the battery module containing the electronics and where the wires attach is thinner than the rest, providing a space about 3mm wide along the narrow edge of the battery that sits above other components on the board.
My asseessment is that there may be space for a slightly larger battery if it was not rectangular, but this would require careful design and provide only marginal benefit.
Thanks for the guide. However I found several steps at least misleading, or not directly applicable to my Moto 360. I'm not sure if this is because the Moto 360 internal construction has varied over different production iterations.
The key difference is this guide implies that the inner body can simply be slid up and out of the metal shell with slow and careful prying. Actually, there are 4 curved plastic parts (call them locking pins) that fit corresponding slots in the metal shell and the plastic inner body at each side of the top and bottom clasp attachment points. Once inserted during manufacture, there is no way the body will ever slide up and out of the shell. I had to apply significant pressure to bend the shell out and pry the body out past the locking slots. However, it's possible that the locking pins could have been removed with tweezers permitting a less energetic disassembly of the watch.