For your purpose I suggest using an amplification optical device suited tou you size of slab that may have projected wthin a measure grid. If this is of no avail, traditional means of measuring cracks are pocket calendar size plastic transparent sheets with straight lines printed in that you superpose to match in sight size. I have one in front of me edited by our professional chapter, the Colegio Oficial de Arquitectos de Aragón in Spain (er, I never thought I would have any use for it). Instead of sending it to you, make the same in one Pagemaker or so and Print in one plastic sheet yourself...or a printing service can do it for you.

I believe the slab size you have chosen is much too small to effectively study cracking in concrete.  Even if fully restrained, you will likely not build up sufficient stress to induce much cracking.

I would suggest long, narrow strips of concrete to obtain appropriate cracking for study.

Use an optical comparator device or mount crack monitoring devices directly onto the concrete.  You can also monitor electronically with crack monitoring devices (LVDT's spanning the cracks), and you can monitor drying stresses with strain gages.