Ready made curtains usually come in 3 widths 46",66" & 90" . You need to measure your rail or pole and get the width of curtain nearest that measurement. You do not need to double the measurement because the sizes quoted are for each curtain and as you normally buy curtains in pairs you have double anyway. If you get them too wide you'll have far too much fabric to draw them back properly. Lengths are normally 54", 72" & 90" .Measure from your rail/pole to the sill you need at least this measurement possibly an inch or 2 more if you are buying curtains with 3" tape. The length is more a matter of preference if you like longer curtains then you can get the longer length.