All is not lost. One of the really great things about working with PVC pipe is that it's easy to cut and glue. So in this case, if the strut was too long, cut it someplace in the middle and measuring carefully, figure out how long it needs to be using a female/female connector from the hardware store. Once you glue it, it will be just as strong as the rest of the struts, only with another joint in the middle. If you put too short a strut in place, you'll need to add length, so you may need to use two female/female connectors to splice a piece into the middle