Size depends on the species, but untill recently we had an asian softie call Bob that had an 18 inch 'shell' and was nearly 2½ feet from tail tip to nose.
She ate precessed turtle food, chopped liver, cooked chicken, sprats and occasional watercress or spinich.
Softies require absolutely perfect water as even just a little amonia can kill them and they prefer to be able to burrow in sand. They MUST be able to haul out of the water entirely to bask under a specialist reptile basking lamp.