I have radiant heat in my house, but the upstairs room (approximately 11' X 19') is a little chilly. The construction is new 2X6 walls with good insulation. There are many windows, however, and the ceiling is cathedral. I am interested in raising the temp about 5 degrees or so for when I am spending time up there. How much wattage in a heater would I need?

Because of the cathedral ceiling what heat you have is rising up into the high space. Ever thought of paddle fans? A twenty foot long room can use two.

They do help greatly for high ceilings to push the heat down to where it can be used. Being new with so many windows I hope the windows are energy efficient.

Any electric heat will be dry and rise just as quickly. Adding humidification will help make the room feel warmer.

In the northeast, 11 watts per square foot of electric heat is a constant for 2 x 4 insulated walls, and 9 watts is used for 6 " walls. To compensate for the high ceiling and high heat loss of the windows, I would use the 11W/SF, either a 2000 watt or 2500 watt heater would work well to assist the radiant system.

Again, because of the length, two heaters would help distribute heat more evenly. This would be two 5 foot 1000 watt units.