If you buy a pack of curtains 66" wide then you will get a pack of 2 curtains, and the will each be 66" wide. So the total will be double the width of your window.
This should be sufficient gather for you windows.
You would have to buy the 54" drop to fit your windows, as this is minimum standard length that will be long enough.
Curtains usually come in 54" drop, 72" drop and 90" drop.
If you wanted them to be very full and gathered across the width, then the next width size up is 90" wide, but that width usually comes with a 72" or a 90" drop only. So you would have to shorten them or have your curtains to the bottom of your radiator or to the floor.