Usually over the largest part of the bust. That can sometimes make the sweater too large all over if you're large there and not anywhere else. So in that case, measure the 'high bust' which is at the underarms and if you knit to that measurement, it will give you negative ease, or a little smaller. I'm pretty big (though thick in the middle too) and I knit to about 2-4" less than my fullest bust measurement. The knitting stretches a bit without looking too small, and I don't need waist shaping as the knitting conforms more to my body without being too baggy.
sue- knitting heretic