Well, I've done some tuning down to F#, and I would recommend
at least a .070. Probably though, a .074 or .076 would work better. You may even try a .078 or .080. If you plan to use one of the larger gauges, though, be prepared for some difficulties in stringing the guitar up. You might have to get the hole in the tuner reamed out a bit to fit anything bigger than a .068 or so in there. Also, if you have an
Edge trem, you may have problems getting the string in the saddle. I don't know what you do about that.