HELP! What size flexwatt for 46" long aquarium?

What size piece(s) of flexwatt should I use for an aquarium 46" long 12" wide and 21" height? should I have one on he cooler side and one on the basking side?

That's a long tank. You'll probably want to cover half to 2/3 with 11" flexwatt and keep it at 82, and cover the other half or 1/3 with 11" that stays at 90-95. Unless the room temperature where the tank is is 80 degrees you'd probably never get a good thermal gradient with out covering the entire length of the tank with two different temperatures strips.