Flora what size do you need that's proving so difficult? I promise I 'm not advertising but I really suggest that you look at the elmet site where they offer all the standard widths, free alterations to your length and both eyelet and pencil-pleat styles. Plus advice on requirements and measuring. If you buy curtains which are too narrow, you can make them fit but the pleats will be too spread out and your curtains will look skimpy. Even if you find they have nothing you like, it's quite a helpful site.
Curtains arrive flat.
On the carpet, mark the width you want your curtain to be (approx half the total length of your curtain rail).
Lay a curtain flat on floor.
Find the strings at one end of the heading tape and knot them together.
Find the corresponding strings at the other edge.
Pull the strings, forming the pleats as you go until the gathered heading is the width you need.
Insert your existing hooks (then you know they'll fit the rail) into the little gaps in the top, middle or bottom row, depending on distance from rail to floor.