I have Regular(10 13 17 26 36 46) gauge strings on now but they feel too small. What would be a good gauge for C standard tuneing?
I use 10-52 DR Dimbag High Voltage. They never break, they are still thin enough to be crunchy (60+ low e strings sound awful in my opinion, nothing but mud). DR's will last forever and stay nice and bright.
Peavey XXX 40efx
Schecter C1+ (sd 59bA4 neck/sd Custom8 Bridge)
Washburn Idol WI45(AirNorton/CrunchLab)

ESP/LTD Viper 300(EMG81b,89n)
Boss SD-1, Crybaby, MXR 6-band EQ,
Digitech DL-8 Delay Looper, SupaCharga
what scale is the guitar you are using?

likely 24.75" or 25.5"

longer scale helps the tone of thicker strings a lot.

Anyways, I use .12-.56 (I think .56) with the wound G for D standard, and they're pretty good. They should work lower but I'd go for .13s if your guitar can handle it.
C standard, thats basically Drop A you're looking at there.
Ted's spot on, 12-56. I use 11-50 for Drop c/d standard, and it handles C# standard and drop b well.