It's a little hard to find on the AirAsia website, I found it by typing "baggage" in the search box and got this:
"You are allowed one piece of baggage onboard and/or 1 laptop bag or 1 handbag on-board, each of which should not exceed the size dimensions and do not weigh more than 7kg when combined with the cabin baggage. Our overhead compartments in the aircraft cabin are built to hold a certain size and weight of baggage. Anything exceeding these dimensions put our guests at risk of injury from falling objects should there be turbulence mid-air.
Size Dimensions: Hand-carried baggage should not exceed dimensions of 56cm X 36cm X 23cm.
Weight: All items must be placed inside your main hand-carry baggage and NOT weigh more than 7kg COMBINED.
Such baggage should also fit under the seat in front of you or in the overhead compartment in the cabin. The approved cabin baggage must be tagged with a cabin bag tag.
You are also allowed to bring along your musical instrument which is packed properly and must not exceed the approved weight and size as stated above so that it can fit within the overhead compartment. Combined with your main hand-carry baggage, all items should NOT weigh more than 7kg.
If your musical instrument is larger than the stated dimensions we would advise you to buy another seat to safeguard against potential damage in the cargo hold."
It sounds like the total weight of your baggage can't be more than 7kg and ultimately have to all fit in one bag, which in turn fits within their dimensions: "All items must be placed inside your main hand-carry baggage "
Hope this helps.