Any pics and current dimensions of pot and plant? Don't agave grow slow in which case you don't have to go that much bigger in pot size unless you are putting the pups in with her? Or if you separate the pups you can plant pups in the old pot. Sometimes the root ball can tell you how much room you need to provide. Some of the biggest plants have the smallest root ball. Some plants also grow bigger with bigger pots. Sometimes, you can control size of plant by size of pot you plant it in.
When repotting: If too big, ask somebody to help you. If prickly and sharp to handle, use towel or blanket to cover sharpness, allow to lay on ground and prevent breakage of leaves and also to maneuver when hoisting or placement.
