OK guys, help me out here! I wanna tune my guitar down to DAFCGC - drop C! What strings should i use!!??? What size ?????
Generally for that tuning you'd want a heavy set of strings to accomodate the lower tension. Start with 11s, and see how they feel.
There are strings out that are specifically made for drop tuning. Generally they have a lower, deeper tone and start at .11 for the gauge. They are called X Rockers. I have them and they are pretty good.

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Id get som Elixirs, mainly for long life. You can use any gauge, The Sword (awesome Sabbath-y metal band) drops to C standard and uses .09s, but Id go with .10s at the very least, probably .11s.
go with .10's. That's my suggestions. Whatever you use right now (unless it's thicker than .10's), slowly transition down.
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OK, so what do you think about taking the Ernie Ball - Skinny top heavy bottom!!??