Our ISBN size is 1.25" wide by .7" tall.
If you are having someone else do the design, it is better to simply leave a space in your artwork. When you upload your file to us, you will be able to place the ISBN yourself during that process. There, the ISBN will include its own white box, so there’s no need to include one in the art (and having a white box in your art may actually make it harder for you to line up the code correctly, as you can’t zoom in during the placement stage, so there’s a good chance of ending up with offset corners). If you simply leave an obvious space for the code, that will make things far easier for you to drop it into the right spot.