Alright, so my neighbor is interested in getting a aquatic turtle but wanted me to help him figure out which one stays the smallest.

I am not smart when it comes to turtles, so does anyone have any idea what Aquatic turtle stays the smallest?

terrapins? perhaps spotted turtles?

Eastern Mud Turtle.

3-stiped mud turtles stay very small and are very aquatic, as are most of the mud/musk turtles. I have lots of baby 3-stripes as my female lays 4 eggs every month like clockwork.

Adult painteds dont really seem large or difficult to me. A male southern would probably not exceed 5 inches, and they are inexpensive, readily available, and pretty too.
I dont know where in Il you are, but there is a guy in Kenosha WI that breeds Heosemys spinosa and they stay small, ~4'' but they are semi-terrestrial. I think He charges around $100 for babys. Send me a pm if you want his contact info.