I am in the process of replacing Federal Pacific baseboard heaters and thermostats because the thermostats stick on and some of the heaters are no longer functional. They are rated 240v/16a and vary in length from 4 foot to 10 foot. Using an online calculator (http://www.ehow.com/how_5252287_calculate-size-electrical-baseboard-heater.html) to calculate wattage needed for each room I have come to realize that I will need at least 3240 watts to heat a second floor room 18' x 18'. The current baseboard unit is a single 10-foot heater and I would prefer not to rewire for two units if possible.

I believe they do not make a 10 foot baseboard heat any longer.

You can find 8 foot in 2500 watt units, see:

The Cadet Electric Baseboard: The practical choice for bedrooms

Then what you are saying is that there is no way to do this without rewiring using more than one heater unit?

If you need 3240 watts to heat the room, and the largest baseboard unit is 2500 watts, you will need more than one unit.

There are other types of heaters,

The CEH: Industrial Unit Heater

That will work if looks do not matter.

Thank you tkrussell, one last question/request... would you mind giving an explanation about the differences/simlarities for the AUBE TH115-A-240D 3600 W @ 240 V, 15 A and the TH115-A-240S 4000 W @ 240 V, 16.7 A? If I understand correctly, one is a single pole and one is a 2 pole? It appears that using a TH115-A-240S thermostat with my existing FPE 10ft 240v/16a baseboard could solve my problem?

How is a thermostat going to solve your problem of using one heat unit in place of two?

I apologize, what I am trying to say is that I could make use of the existing baseboard heater and just replace the thermostat. The existing baseboard heater is a FPE 10ft 240v/16a unit. The total wattage of this unit is 240v x 16a = 3840w and sufficient for the size of the room. Using the TH115-A-240S 4000w @ 240v, 16.7a thermostat sounds like a viable solution in this case. My confusion is with regard to double pole vs. single pole thermostats. Does the single pole thermostat only open one leg of the 220v circuit and is it safe? The description shows 4 wires exiting the unit. http://www.aubetech.com/manuel/2/TH110-SP-P-240.pdf
Again, please accept my apology for not being clear.

Use the two pole thermostat.

A single pole is allowed to be use in a residential application, but, your correct, it only opens one leg of the 240 volt circuit.