I'm a 34-D/DD when nursing, with more of a belly left after pregnancy two than pregnancy 1. I have several Boob size large tops (short sleeve t-shirts), and they're fine. And one medium, which is also fine -- but it's cut looser through the belly. If your waist has returned, I'd say try medium. But they can very from top to top -- different designs are honestly different cuts.
They do shrink a little with washing, even on delicate and low dry.
But, they are totally the best nursing tops!
OMG! So stretching and so convenient... I remember some of the awful Motherwear things I had last time around... I do not get the tops with the floating partial underlayer at all.
(I also wear L in Glamourmom tanks, if that helps... I think both brands are sized pretty small overall.)