I'd go with 11's or 12's on 25.5 scale for that tuning. But get a thicker string for the low string. Drop A# is C standard with the low C dropped to A#. I increase the gauge by 1 for every full step down. C is 2 full steps down from E.
If you like the tension of 9's in E standard get 11's for C standard. If you like 10's in E get 12's in C. Then increase the thickness of the low string for the A#. If you decide on 12's for example then get a single low string from whatever comes with 13's to use for the lowest string instead of what comes with the 12's.