So I built a touring bike with BB7s. While I was readjusting them (its winter and I am bored) I notice the rear rotor is ever so slightly out of true. Seems like a good reason to spend some money so what is the determining factor on what size rotors I use. Can I intermingle brands, say a shimano or hayes rotor. Dont know if it matters but its a Soma double cross frame with the IRD strait blade disc specific fork.

If you change rotor sizes you'll have to acquire some kind of adapter to make your caliper match.

As long as the pad is completely on the braking track of the rotor, mixing brands is not a problem. I've done several builds with Shimano Centrelock discs and Shimano rotors and Avid calipers. Works fine.
However, warped rotors can be trued, and new ones out of the box often need truing. Why not just true the one you have?

how??

bend it

If you have a small adjustable wrench, you can adjust it until it just slips over the rotor and gently and gradually bend it into shape. Be sure to thoroughly clean the jaws of the wrench first.
I have a Morningstar rotor alignment tool set, but in the absence of that, an adjustable wrench can work.
OR, it's a 5 minute job at the LBS if they have the equipment.

If you have a truing stand then set a reference, to judge where the disc needs to be bent.
C clamp something you can fix in place close to the disc.

+1....my goodness, it's rare to set up a disc brake system without having to do a little truing of the rotors, they're usually out of true right out of the box. Just use an adjustable wrench to true it.

Well, fair enough then. Thank You all for your help. I guess I will be learning a new trick this weekend

If you do decide to go with a different size rotor then beware that it will move the brake a bit, so make sure that this does not then interfere with any rack that you have on the back of your tourer. I actually chose to use a bigger rotor on a bike I recently built for a friend so that the brake sat in a more convenient location.

i use the pliers trick, but wrap some thin corrugated cardboard between the rotor and the pliers before i make the (gentle) bend. I also mark the bend areas by simply rotating the wheel on the bike and using the pads as the true points, then marking bend areas with a sharpie and try to bend at the branch (and less at the unsupported rotor area between the branches).

The results: super cheap/free, super simple, and my fixes thus far have been 100%. hard to beat.

As for rotor size on the rear, if you're not hauling a loaded trailer or dragging on a tandem, a larger rear roter will only mean skids and therefore more tyre wear. 160mm is more than adequate, even if you lose "wagging" rights.

I true my disc rotor with my hand (while holding a rag to prevent oils from getting on the rotor).

Agreed. I'm running 160mm rear, 185mm front (Avid BB7) on my Big Dummy, and even fully loaded with my two grandkids on the rack, (and a clydesdale grandpa up front),I've got lots of stopping power.

Dan, that quote in your signature line is awesome.

Thanks. Nobody writes like that any more.