Dependent on scale, build etc. for a drop A I would use a .135-140 B, then normal gauge for the rest. If you are dropping to F#, you will need a much thicker string which will result in very high tension when in drop A.
If you are planning to drop to F# a lot, I woud slap on a set of Kalium 174 136 106 79 59 43 and then tune (as standard) F# B E A D G, for drop A you then just drop the B to A...