Hi, patricianwill!
From Data tab, Data Tools group, Data Validation icon, you can set the length allowed for that cell, but I think there's no way to perform the auto tab action, because cells haven't events like KeyDown, KeyPress or KeyUp as text box controls in user forms do, they just can handle the Change event which is triggered only when you press Enter, use an arrow key, clicks on another cell or menu, ... that's to say when you manually do what you intend to be done automatically.
Regards!