Depends on the feel you want. You can have it set up for almost any gauge. For drop C I would go with an 11-54 personally but I cannot stand flappy strings.
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Think my Carvin is 25" and I've played in drop C exclusively and have enjoyed 12-56 (a bit tough on the fingers, but really punches) 11-54 are awesome, 11-50 worked, 10-52 is what I think I currently have on there and I'm digging that too.
11-49. I like to bend my strings like gumby. Vibrato on point, easier to play and sounds great.
You will just have to try and see. Good thing about fixed bridge is you can buy 3 different gauge sets for $15 and not need to spend as much time on a setup. Just try a few and see what works for you.
There are also tension calculators online so you can plug in what you have now and then compare to the others. You can drop tuning and still get the same exact tension you have now (or very close) with different gauge.
11-54 or 52. Personally, I got used to 10-52 on a 7 string, as that is how it was set from the factory. In standard D tuning, the 10 is too flabby, so the 11 is just right.
I typically just use 10-46 since I am normally in D and only drop the C sometimes, but I would probably try 10-52 if I was always in drop C. I actually just bought a set to try in C# tuning on a 25" guitar. I like light strings though.....