Great sizing advice here.
I can only add that I prefer to use at least the 12x9x6 box. The 7x7x6 boxes are quite popular, but there is not a lot of room for error there. And if you are doing more than a couple of shipments, then def buy the components separately and in qty, it is more affordable that way- a (small) bunch of boxes, reptile bags, heat packs for the coming winter season, etc.
And use coupon code APS40 for 40% off the FedEx retail price for your first shipment!