I was going to say the same as Kmn,
You'll find the bigger sizes on Maternity bras have wide backs anyway.
I suggest going into a store to get properly fitted, The women are pretty spot on with sizes. When I got mine fitted while pregnant they made them slightly too big, which was awesome for when my milk came in. Remember it will only take a couple of weeks or so and your milk supply will settle down too, so they won't be as huge..

Only thing is price, geeze they're expensive! Mine have all been about the 60 dollar mark

hth