I have a gorgeous pair of long drop (2.67m) Designers Guild Ombrione curtains with triple pleated heading that are 2m wide each (including the effect of the pleats) that were in a bay french window in my house before last.
I now have a fairly standard Victorian double living room, with a bay at the front and a small French window at the back. I was thinking of getting these curtains altered so they would fit both windows. They wouldn't quite be as wide a set of curtains if you were starting from scratch in that thy might not cover the whole window frame but I only need them for 2, maximum 3 years until we finish a side return extension and decorate the living room, which including taking the current PVC windows back to wooden sash again would probably mean I would have plantation shutter at the front.
The drop of the new French window is exactly the same as the curtains which is handy but as there is a radiator currently running under the bay, they bay curtain would have to reduced to 2m in length.
I have no idea where to find someone to do these alterations (Google has not been my friend here) and the cost, as I don't think it will be the simplest job.
I could sell the curtains of course and put the money towards getting some made to measure (as I can't find ready made lined curtains of the right lengths) - a quick estimate on John Lewis 7 day curtain service puts that at about £7/800 using fabric at about £16 per metre.
I don't know how much the curtains are worth second hand. They are in very good condition in a neutral fabric that is still in production and almost iconic - when I had them made, about 8 years ago, the bill for the fabric, the curtain rails and the fitting came to about £3000 - I think about £2200 - £2500 was curtains.
Of course, I can still sell them after I have them altered but I don't know whether 2 smaller sets of curtains would be as valuable as one really large set.