The lining piece will be the size of your pocket less the size of the facing piece, plus a seam allowance. (bygumbygolly.com)
I'd rather see more items in less sizes then a small selection, but lots of sizes to choose from. (thefashionablehousewife.com)
Then it might help if you also check if the pee pads you use or those that you find economical would be more or less this size so you need not fold or cut. (topdogtips.com)