If you have or know someone with Electric Quilt, that also makes it easy to paper piece any size block and is perfect for times like this. You design it any size you want and tell it to print out what ever size you want at the end.
Peckish describes the math so well! I don't do math in my head any more but from years of working in offices I know how to find where to make it happen on copiers, I actually do more quilting than people would think at the copiers at my local Staples branch. You might even have good options at home if you have a multi-function printer/scanner.