Hi Johnny,
I'd recommend going all the way to 13s for C or C#. I used C constantly for about a year or so on a 25.5" scale guitar, strung it with Rotosound 13-56s and they still felt pretty slack, probably like 9s would do in E.
One thing that might limit your choice of gauge is the width of the bridge and nut slots; I used 13s with a Floyd so it wasn't a problem, but I think standard bridge/nut slots may be too small.