Hi, Welcome.
Is this quilt to wrap the baby in papoose style - or burrito style? (Hope I'm not offending anyone here) Then square works well. 36 to 45 inches.
Or is it to go in/on a crib - then maybe 45 x 60?
Or in a bassinet? Don't have any idea for sizes there.
I preferred square receiving blankets for my babies - but that was when the dinosaurs still walked the earth and before onesies.