Hello Pamela!
Your quilt is looking beautiful, what a nice idea to use your son's baby clothes;
If you've done 5 x 8 squares and they've come up 41/2inches finished then your top should measure 22 1/2 inches by 36 inches - I would measure it to be sure though. So basically you need 4 border strips of whatever width you decide (plus 1/2 inch seam allowance). On border width, i like to chose a size that somehow relates to the block size, I think it always looks better that way but that's just personal preference, so for your block of 4 1/2", you could do the border the same width, or half the width or 1 & 1/2 the width, depending on what size you want to end up with. THEN you have to decide if you want a lapped border, mitred or corner blocks. I'd go for lapped on a first try, it's the easiest! You need two lengths the width you want (don't forget seam allowance!) times the length of your quilt. Sew these on each side. Then two more pieces the width of your quilt PLUS the borders you just added. If your quilt is 36" long, you can certainly cut the borders across the width of the fabric (it's usually 44" wide), if you go for 4 1/2" wide borders, then your quilt should measure 31 1/2" wide (plus those seam allowances!) So you can still cut across the width. Therefore you need to buy 20" of fabric (I'd just get a yard - leftovers are never a problem once you get the bug! :wink:
I hope I've been helpful, I'm not very good at explaining :oops: I'm sure someone else here will do a far better job!
K x
PS You also need to think about binding it when its batted, backed and quilted and if you want to use the same fabric as the border for this, you'll need the circumference of your finished quilt plus a bit extra for joining seams x 2 1/2" if you want to double bind, which I would strongly recommend - it's easier to apply and harder wearing! If you don't know the method - just ask when you're ready to go! :D