As far as how much water it holds, if you look at the picture...all the area, except where the battery compartment is, holds water. For reference, they're a little bigger than palm size.
As for leaking, neither of ours leaked on the first trip. On the second trip, mine leaked once, but I had been fiddling with the stopper so it was probably user-error.
We keep ours in our bags and pull them out when needed. The "blades" on the fan are foam and can bend up in your bag. If they get bent or the fan isn't blowing strongly, take each blade and bend them away from the mister and squeeze the ends together, its all fixed and works well.