Quilts for Kids lists 36 x 36 as the size for preemies quilts.
On the other hand, here is a pattern for a smaller one:
http://www.azblankets4kids.com/Patte...20reworked.pdf
Here is a bit of info on preemie quilts discussing various sizes. I believe this blog belongs to one of the QB members.
http://with-heart-and-hands.blogspot...ie-quilts.html