We always tell everyone that we need a measurement reference for all Fossil photos.
Here is a link to a PDF printable mm/inch reference sheet I found on line. I like the setup of this one and have printed it up on a light gray, heavier paper, to use as a neutral background with measurements. I find a lot of rulers, etc obnoxious in photos. I hope you like this one. Plain white paper is fine also, but in IMHO gray will work better.
I also put the link for the search page also if you would like a different style.
http://www.vendian.org/mncharity/dir3/paper_rulers/UnstableURL/rulers_cm_in_narrow.pdf
Search page.
http://www.vendian.org/mncharity/dir3/paper_rulers/