10's will surfice, (I normally use 10's and have been known to drop down to C - even got down to Bb once, but that was pushing it).
Personally, I'd recommend 12's as they'll give you enough tension to produce a "clean" sound without sounding too loose; although it very much depends on both your guitar and the sound you're after.

Experiment with you're current strings, if you're not happy try going up the gauges (I've never tried with anything below a 10-52, although I'd imagine it wouldn't sound too good?...Anyone?!) until you find something you're happy with.

If you're tuning down, the tension on the neck shouldn't be any (or significantly) different from standard tuning with lower gauge strings; I've never had a problem, but then again I don't regularly use the heavier gauges.

Hope that's of some use.