John,
Lots of players have used different guitar body styles for playing altered tunings - from parlour (especially in Open D and Open G) to GC, OM, SJ and up to dreadnought sized. It depends a lot on your preference and playing style. If you are going to be using C tunings a lot then you need a body size that has a big enough body-depth to support the low notes but with choice of scale length and string gauge you can easily use a Grand Concert size. OM is a very popular size with altered tuning fingerstyle players - and fan-fret scale lengths work nicely too - longer scale length for the lower strings that are tuned down. As Colin said I like the fan-fretted guitar I made and play it a lot in CGCGCD and CGDGCD tuning as well as DADGAD. This is a real "strange one" though - 26"-24.8" scale length, 15 frets clear of the body, 16" lower bout width deep body but with a Manzer wedge. The thread on it is
here. I use 12-52's on this guitar and they work well for my style of playing.
For 25.5" scale length then for the bottom string a 0.059" would be a good compromise for mostly C with the occasional trip up to D, and if you were going to be permamnently in C then maybe even a 0.062" or 0.064" would be worth considering.
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Dave White
De Faoite Stringed Instruments". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010