In Flames guy's guitar string gauge, anyone know ?

If you're going to tune to C, I'd suggest trying 13's, anything lighter will feel loose and floppy.

If you're really concerned, try 11's first, and if those are too lose, go to the 13's. All the guitarists I know that are in C use them. I would assume the In Flames guy would too.
 
Yeah, Xeno's right. I tuned my main axe, an ESP with a locking floyd rose trem system, down to C, but the music store I was at to buy strings didn't have anything past 11 gauge. I ended up getting a set of hybrid dean markeley's, with heavier gauge strings on the bottom and lighter gauge on the higher strings for easier soloing. Unfortunately, the gauge was way too light for C, so I ran into the problem of fretting out past the 19th fret on my G-B-e strings, or Eb-G-c in C tuning anyways. Make sure you get your guitar adjusted appropriately for the tuning, as your going to be putting a lot more stress on the neck then its used to.
 
i usually use 12s for C-tuning, but 11s should be ok, too. It mainly depends on you, I think. The best would be to try different gauges to find out what fits you best. I even played in C-tuning with 9s and it worked pretty well, too.
What tuning did you use before, and which gauge did you use for it? (I think this info could help us to help you ;))