All poles made for curtains don’t come bearing the same lengths. This means that you will have to work out the length of the window first before selecting the right pole. And in taking the measurements, there are a few things to consider.
Since you might not want the curtain to block out light during the day, you can add a few centimetres to each side just so that there will be enough space to accommodate the curtains and the linen when they are pulled back.
Also, if you have curtains made out of heavy material and a thicker lining, then it’s obvious they will consume a lot of space when they are pulled back so having a long pole, in this case, is necessary.
If you fail to get a pole that fits well into your measurements, then you will be better going for made to measure/customized curtain poles.