As close as you can get to 45" wide, you just get the width based on the window width. With the drop you have to account for the fact the holding rail will be a few inches above the window and it depends if there is room to spare below the window. I would say your best bet is 46" wide with a 54" drop.
Yes you do have to have folds or gathers, that's why it's usually 2 and half times for each curtain. I used to make my own and line them so have bought loads of material to make them...and curtain pole should go at least 4 inches if poss wider than the window frame.
Fezzy, generally I find if you buy curtains the width of the window then it's fine. It's the drop that is the main worry. If you buy curtains 46" width by 54" drop, that means that each individual curtain is 46"x54", it is not the two combined.