Janet, I order my Baltic birch plywood "good one side" 12"X24" pieces. You can used 1/8" or 1/4" thickness. I like 1/4 " thick better, but I make my own frames out of 3/4" thick stock. So having room for a backer is no problem.
If you are using picture type frames you buy then you might want to use 1/8" thick BB. Order some of both 1/8" and 1/4" and try it.
Most premade picture frames come in a variety of sizes 5x7, 8x10, 8 1/2x11, 9x12, 11x14, 14 x18, 16x20, 18x24, 20x24, 22x28, and 24x30 inch.
You will have to decide what size frame you want or is available and cut the BB to fit.