Hi Tony, 100 x 100 should be enough for maybe 2 necks, that is neck and fingerboard. How are you going to cut the block into smaller pieces ? Been a while since I did a scratchy so can't remember fingerboard and neck blank thicknesses off the top of my head. A rough guess a fingerboard blank would start about 8-10mm thick and neck blank think ends up being about 20mm thick at the heel so blank maybe 22-24mm thick. Any offcuts you can save for the headstock wings you will need to glue on.