I have 7 kids and i grandbaby (!!!) and really, any smallish size will do. I think a 36 x 45 or 45 x 45 would be great.
Just a note, when my daughter had her showers, she got LOTS if onesies, diapers, t-shirt type things, but the quilts got the most "OOO"'s. Not ONE frilly fluffy dress!! (If I didn't have 2 kids getting married plus us moving, I would've made some!!)