Common musk turtles, or mississippi mud turtles are probably among the smallest turtles you'll find in captivity. There are other species of mud and musk too that rarely get past 4" or so. Painted turtles are usually in the range of 6 inches or so. Also making them more managable than sliders or cooters.
Most of the smallest tortoises are endangered, but the smallest easily attainable ones are probably greek, russian or hermann's tortoises. They average somewhere around six inches or so. Egyptian tortoises are smaller yet, but not as commonly seen for sale.
Technically, the 4 inch turtle law should apply to all these guys too... kind of a sticky situation.
There is a fine line between a hobby and a mental illness.