The easiest thing to do is keep to the pattern directions and stitch number for the top, part of the sleeve. That way the decreases at the armholes and shoulders will match the raglan decreases for the back and fronts. (This is just repeating what Jan mentioned.) If you like you can cast on fewer sts for the sleeves and increase at the edges more to widen the sleeve to the expected number of sts before the raglan decreases.
That said, it is helpful when dressing a baby to have wide sleeves rather than more fitted ones. How many times you have to put your hand into the sleeve to adjust clothing!