How old is the baby we're talking about, 3 months?
The wattage of any bulb will depend heavily on what temps you want to reach and your home's/room's ambient temps. If you're not a heavy AC user then a smaller wattage will get you to the 80-84F, but if you're always running the AC very cold then you may need a higher wattage to get to the same temp.
I will say that I don't use more than a 60w halogen and I get mid-80 temps in my cages. So I would see what a 40w is doing and go from there. It may take a little experimenting or raising the basking light up a little off the top of the cage.