As described by Commander Erica Stanley:

She's about 30 years old, is of average height and weight and has brown hair and eyes. 

A little bit on her personality: she's Sam Carter Lite.  She has the same passion and drive for her career that Sam has but not to the extreme that we've seen in Sam; i.e. Jack having to tell her "Ok, that's enough," when she starts to get into tech heavy explanations about some thing or another. 

Since Starfleet does not seem to have pursers on their ships I view her more as a scientist, rather than a command-line officer, as I also hold the position as Chief Xenobiologist.  Being that I am on the Command Staff of the Max I interpret that as Sarah having some command experience, enough that she is able to take watches on the bridge as need be (i.e. the occasional night watch).