If you are going to use poles for square bay windows think carefully about how you want to the curtains to hang.
Do you want to have one curtain on each side of the the bay, or a pair of curtains ?
You need to consider the amount of space the curtains will take up when pulled back, how much light will be blocked by those curtains (when pulled back), and if the effect might be to make the room too dark.
Would it be better to have one curtain on each side and pull it back to the edge of the bay? If you do this measure the length of the bay at each side and double it this will be the length of pole you need to buy when using one curtain on each side. The pole needs to be in one length because the curtain needs to be able to cover the whole length along all sides of the bay.
If you choose to have a pair of curtains, buy the length of each side. There are poles available that can be fitted with elbows so that you can join the poles together. That way, it is possible to have a pair of curtains that can tun the complete length of the bay window. Whichever decision you make about the curtains, choose fabric to suit the room and that you like – not what some else thinks you should have, or what someone else has advised – make your own choice.