I used a 40-watt bulb in my last toaster lamp, anything bigger gets too hot for the plastic bits inside. The real trick is using a small metal bracket to hold the socket steady so it doesn't wobble when you turn the switch.
Turning a toaster into a lamp is a clever idea, and I've made a few myself. First, take out all the old heating parts. Use a simple lamp kit for the wiring, and put the bulb holder where the bread slots are. A common mistake is not fixing the cord inside the toaster shell. A bit of spray paint can make it look nice.