I cant help you with finding what power you'd need, however I can help you a little with the heater calculations:
To work out the wattage of a length of nichrome, say you want 120w, your power supply is 120v, you need to set the resistance of the coil to 120 ohms, so that 120v/120ohms = 1amp current draw, 1amp x 120v = 120watts.
The datasheet for the nichrome will tell you the ohms per foot (or you could measure a 10 foot length), and the fusing current rating (you could deloberately blow a piece of wire on a high current supply, then divide this current by 1.5 or so as a safety margin).
Theres a practical limit to the power you can make a heating element, one is the maximum current the wire can carry without fusing, and the other is the maximum temp the heater coil gets to has to less than the melting point of the nichrome (this can be releived somewhat by your mentioned use of a thermostat).