Usually there are measurements listed for each size- a bust measurement, length, etc. You just measure the recipient of the sweater, choose the size that most closely resembles the measurement, and then make sure you're getting the proper gauge so your sweater will turn out that size.
Mommy to two wonderful girls
"I don't have to conform to the vagaries of time and space, I'm a loony!" -Campbell Bain (David Tennant) 'Takin' Over the Asylum'