Any size from 36 x 45" up to 48 x 60" is a useful size for babies up to kindergarten.
As for block size, I prefer to keep it 9" or under for baby quilts. So, if you're doing a nine patch, cut the strips 2.5 inches for a 6" block or 3.5 inches for a 9" block. I like to make Billie Lauder's, "There's a Dog on my Quilt" for new babies and that pattern calls for cutting the strips 3" wide which makes the blocks 7.5" finished. Very simple quilt that is so cute when it's finished.
http://www.easymade.com She also has patterns for cats, elephants, snakes and I don't remember what all in the newer pattern, "Look Who is on my Quilt."