The size of steam-generator you need is related to the size of your steam box, how well insulated it is and the temperature of the air around the box. It is probably cheaper and easier to insulate your box thoroughly rather than increase the size of your steam generator.
Wall paper steamer: Curtis Buchanan uses two of these to heat a 4x4x70“ I.D steam box. They are handy one-piece units that are readily available from a hardware store.
Propane/electric burner and a pot: Propane burners can work well, just make sure you don’t have too much heat loss between the pot and the steam box. Portable electric burners are too small for this job, but larger electric units can work.
Pressure cooker: Bob Simmons sent me this info and photo. “The image shows my entire rig. Wooden steam box, propane cooker unit for deep frying turkey, and an old pressure cooker. The pressure regulator on the pressure cooker’s aluminum lid was a weighted piece which sat atop a short nipple affixed to the lid. I threw away the weighted “flip-flop” thingee, unthreaded the short nipple, and then drilled out the hole to 3/4” and installed a 3/4” Brass nipple. The steam source connects to the box with 3/4” radiator hose.”
Hot water heater: This is overkill, but works great. Five to ten gallon used hot water heaters are $20 from scrap yards, plumbers and craigslist. Take out the thermostat and replace the element with a 5500w 220v element from the hardware store. 110v elements are also available, but I haven’t tried them. Connect a wire and plug to the element and you are all set. Mine produces a tremendous amount of steam, goes through gallons of water and takes 45 minutes to come to a boil. Make sure to keep the pressure release valve in place – you don’t want to make a bomb!
For further reading: The first part of this series talked about heat, what to make you box out of and whether you really need a box at all. Part two went through doors, extensions, and other box components.
The Wood Bending Handbook is a great resource about bending wood.