Hi. I use the Roger Russel speaker wire size recommendations from the
site here. If you scroll down just a bit, there's a handy table for wire gauge as a function of speaker impedance and wire length.
Another recommendation is to use in-wall rated speaker wire. You can check your local building codes, but it might be required for insurance reasons. You don't need to spend a lot of money. Monoprice makes good wire, IMO, as do others.