On the plus side, plate glass polishes beautifully and fast, equilibriates quickly and is cheap. I have ground successfully the following center thicknesses and diameters:
3/8" x 12" 5/8" x 16"
A friend regularly conducts mirror making classes using bank door glass of 7" diameter and 1/4" thick.
Use the standard precautions - rotate frequently, polish on something soft like a blanket or towel, use reasonable focal ratios, keep your fingers away from the edges, and once the mirror equilibriates, close off the back end of the tube.
Mel Bartels