The smallest land tortoise is Egyptian Tortoise.
The smallest aquatic turtle are musks/mud turtles. Bog and spotted turtles are pretty small as well - but threatened and/or endangered.
I don't think you should dwell too much on size, cause small land turtles are rare and expensive. If you are interested in relatively small land turtle you can consider a box turtle, which are around 6".
If you are interested in a small aquatic turtle, a mud or musk species are pretty easy to get and don't get more than 4-5" depending on the subspecies.