I have two small short necks in a custom made turtle tank with ledge.
The turts that used to be in it are now out in a pond because they got too big for it.
With depth you can have it 3/4 full, if it looks too deep put in some rocks or something for the turts to stand on so that they can stick theirs noses out of the water.
If you can get a tank with a ledge it means that you are not sacrifising swimming around for a place they can get out.
It can be the same as fish, the bigger the tank the better the water quality will be with waste.
Also with turts the filter will need to be at least 4 times bigger then the tank size.
They can be stinky but with a bigger tank it is easyer to keep clean.
Also don't put anything in the tank that will fit in the turts mouths except food. In other words don't use a substrate of gravel, Keep it as a clear base with just big rocks for them to climb over.
The other thing is over feeding, like fish they will eat what ever you put in front of them.
With fish it is apparatly the size of their eye to how much you feed them, with turts it is the size of their head.
Young turts feed once a day and older 2-3 times a week