Four 28"x 60" game bags work perfect for me with rear and front moose quarters on the bone, separated at the knee joint...the front quarter is actually longer than the rear, which I'm sure you already know.
The additional needed bags for boned-out meat can vary in size based upon other individual factors, e.g., the weight of the loads you want to haul out on your back, your preference on separating back-strap/tenderloins/heart/liver, and then rib meat on the bone or boned-out (based on your preference and regulations of the GMU...it varies in Alaska, some require rib-meat go out on the bone, some not).
Since you're looking to buy reusable bags, I'll mention that I'm personally a big fan of TAG Bags...light and durable.
Edit: My comments were applicable to Alaska/Yukon moose, not sure the size for Alberta moose. Sorry about that...I was too eager to be helpful! Hopefully, others will offer you more applicable suggestions for Alberta moose.