First off, a good size for a baby blanket is anywhere around 32"x42".
Baby Ull has a gauge of 8 sts per inch.
So, you'll need about 256 stitches across. (32 inches X 8 stitches)
Here's how to figure what to cast on:
If your pattern says it's a multiple of say 6 stitches plus 2, then 6 x 43 = 258 plus 2. Cast on 260 stitches. You'll have 43 pattern repeats across the row, plus 1 stitch on each end.
If the pattern says it's a multiple of 8, cast on 256. You'll have 32 repeats across.
If you want to knit a border as you knit the blanket, cast on those stitches too. If you want to add a border afterwards, cast on an extra 2 stitches so you have somewhere to pick up stitches to knit the border on.