To use Time Machine I'd suggest a drive that is at least 50% larger than the one inside your MacBook.
As for physical size, the 2.5" ones would be powered from the laptop itself via USB, whereas a 3.5" "desktop" one would need to use a power adapter.
If you may be away form home and still want to use Time Machine a portable 2.5" based drive would be the best option although a lot more money per Gb than a larger desk based one.