If you're using the type of curtains with the ring set into the fabric then definitely use a double width.
Otherwise when they're closed the rings don't sit straight on the pole and they're awkward. I found that out the hard way!
If you don't want them to overpower a small room then choose a pale colour, but lined, curtain, with a same colour woven pattern in the fabric.
Ours hang just below the window sill and I've fitted a radiator shelf so the heat doesn't escape behind them.