The "Can I Mash It?" calculator here will help you determine what size beers will fit in each. My 10gallon mash tun will fit ~21lbs grain with 1.5qts water/lb. http://www.rackers.org/calcs.shtml
About grain bed depth, if you have some grain, Measure a gallon of grain and weight it to get gallons/lb. My guess is around 6lb per gallon based on some buckets of grain at home.
The volume of those kettles is pi*radius^2 per vertical inch (in cubic inches) - or -
Small: 6.375^2 * 3.1415 = 127.67 cubic inches = 0.55 gallons/inch
Large: 7.875^2 * 3.1415 = 194.82 cubic inches = 0.84 gallons/inch