This is not a Miscellaneous Non-Financial Question.
If the window recess is 5' wide then you will want a pole that extends beyond that on each side to some extent. No hard and fast rules about how much because it depends on the window, room, curtains, finials (the yokes on the end) etc. Normally you buy them bigger anyway and cut to size unless they happen to be exactly what you want.
Most poles (except timber) are adjustable - where there are 2 parts and 1 slides inside the other. Michael Guineys are one of the cheapest for poles. Bear in mind, most poles are measured in cm and as already stated make an allowance for an over hang on either side. That way you don't block the light from the room by having the curtains hanging in front of the wall as opposed to the window!
We bought really nice chrome ones in Argos. They are all telescopic - you can me them as long as you need as one sits pole into the other. You have to buy the finials separately for the ends of the pole. We got these in Argos too. They were really cheap and cheerful and look great