I've found they run a bit small, although we've just ordered the Bitty Baby matching clothes so far (I think they call it the Little Girls Sizes) My daughter is 46 inches, 40 lbs and usually wears a size 5, and the little girls size large (wihich is 6-6x) fits just fine, with not much growth room, if that makes sense. I'd suggest going up one size probably. Luckily, with a dress, if it's a bit big it's OK.
That new dress is adorable! My daughter just got the bitty baby twins for Christmas so she's not getting one of the bigger dolls for awhile - too bad they don't make this in the bitty baby size too!