I would think a 50w would be fine, assuming you get a high quality heater that turns off when it reaches its desired temp. Otherwise, there are 25w heaters out there. In either case, the Cobalt Neo-Therm line of heaters is recommended a lot (pricier compared to glass heaters, but no worry of breaking).
No matter what option you go with, a temperature controller is also something to look into. Can't speak for any particular brand myself though.
No matter what option you go with, a temperature controller is also something to look into. Can't speak for any particular brand myself though.