I use a 3L platypus with my 44 mag. They fold up/ compress very compact. I usually hike in with a full Nalgene then fill the bag when I get in and dump camp out.
Camelbak Milspec 2 and 3 liter (short-wide version) have served me well. The twist seal cap on the bags are a little finicky, so make sure you don't have any leaks before you put in your pack