13 X 16 works out nicely, in my opinion, for 4X5, 5X7, and 8X10. This allows six mounts and/or overmats to be cut from a 32X40 sheet of mat board. I don't enlarge larger then 11X14 much anymore. I use 16X20 mount and overmat for that size print since it allows better utilization of the material again.
For 12X20 contacts I make a mount and overmat 20X30. With the sides and top width at appr 4 1/2 inches and the bottom at appr. 5 1/2 inches. This again allows a mount and overmat to be cut from a 32X40 sheet of mat board.
I use an acid free storage case for my loose prints. I use a portfolio case for my matted and mounted work. Both obtained from Light Impressions.