Dweste,

Sizes vary from drive to drive, PC and laptop drives have smaller magnets, the larger server type drives have larger ones, ranging 1 to 2 inches across, slightly more than 1/16 inch thick and the width varies, I've seen a couple that were more than 5/8's of an inch on the third dimension.

They will generally have some curves or notches on them and resizing them is iffy at best. Mostly just by breaking them in my experience, not real accurate, but if you have enough you can generally break one the right size (eventually). Best bet is to use the flat angled glass breaker pliers (regular jaws that have small pieces of angle iron on them to hold the work piece with parallel grips).

Eugene's commment about the hardness is accurate.

Regards,
Comanche7