In general, shorter cable lengths are better. The length should leave a comfortable amount of slack between the two pieces of gear. More importantly, it should be the correct type of cable. If the mixer output is balanced and the interface input is balanced, you would use a TRS cable. If unbalanced, you'd use a TS cable.
Does your ProTools interface have an XLR mic input on it? If so, I would consider going directly from the mic to the preamp on the PT interface, bypassing the mixer. The idea with that is to keep the signal path as short and simple as possible from the mic to the recording device. The less electronics in the path, the better. Especially cheap electronics, if it is a budget mixer.
But your setup is minimal and direct to begin with, so just make sure you set up the levels properly and you should be able to do some nice sounding recordings.