By default maya uses centimeter as the working unit. It doesn't really matter though, as long as your proportions are correct . Meaning a 1 inch cube and a 4 inch cube have the same relationship to one another that a 1 cm and 4 cm cube have. I normally don't change the working units in maya because different people working on a group project might get out of scale of one another.
Anyway, if you do want to change the default scale you can do so under Window->Settings Preferences->Preferences. Look in the settings under categories.
I've always wondered if the working units had an affect on depth fo field settings for the camera. If anyone reading this knows please chime in.
Thanks and good luck,
ww