Here are a few alphabet charts you can use to sign your work. The heights assume you are using mixed upper and lower case; less space is required for all upper or all lower case.
Your designs are true masterpieces! I have never seen anything like them before. I actually haven't stitched in quite awhile because most of the designs out there are just too simplistic for the amount of time that goes into stitching a picture. Keep up the wonderful work!