If you have a small Kindle, your hexagons should end up pieced together in a rectangle-ish shape measuring 8.5" by 14"-- PLEASE be sure you are measuring from the shortest points, ie the dip in between hexies wide and the last full hexie edge long, NOT any pieces that hang over.
If you have a Kindle DX (the larger one) or an iPad, since it's within a few tenths of an inch in dimensions from the bigger Kindle, you will need to piece your hexagons into a piece that is approximately 11.5" wide by 18" long.
I have no idea how many more hexagons you'l have to piece. Sorry. Just keep stitching until you have these dimensions.
I hope this answers any questions about optional sizing- everything else will be the same.