Curtain size calculator
Type your window's width. You'll see how long the track should be and how much wall the curtains take up when they're open, so they clear the glass. Your installer measures properly before anything is made.
Track: about 3.12 m
Each curtain takes at least 0.36 m of wall each side when pulled right back.
How it works out
The track runs past the window on each side, far enough that the open curtains stand on the wall, not in front of the glass.
Pulled right back, wave curtains take at least 23 cm for each metre of track, and pleated curtains at least 30 cm.
A curtain that opens to one side puts all of that at one end.
If you don't mind the open curtains covering a little of the glass, the track can be shorter.
These are the usual figures. Your installer may allow a little more or less.