Most shower curtain poles or rods are expandable by at least a foot or two. The poles are made up of a wide section and a slightly narrower section that telescopes into the wide section by means of a screw mechanism. Making sure that your shower curtain rods are fully extended is very important, as a rod that's not fully tensioned against the walls of the enclosure might fall down on you.
Tip the narrow end of the shower curtain pole down and shake the pole. The plastic ring or collar that locks most shower curtain poles into place should drop down toward the narrow end of the pole. If it doesn't drop down, use your fingers to gently pry it loose from within the other half of the rod. This effectively unlocks the rod's telescoping mechanism.
Hold the rod in front of you with your right hand on the narrow end and your left hand on the wider portion of the rod.
Twist the wider (left-hand) portion of the rod away from you to expand the rod. If you're going to mount it in a bathroom, keep twisting until it's nearly as wide as the place you'll mount it, then hold it up in the position where you want it mounted as you keep twisting.
Keep twisting the rod until it's as wide as you want. For mounting purposes, this means that you should twist it until both ends are pressing firmly against opposite ends of the shower enclosure and you can't twist the rod any further without exerting great effort.
Slide the plastic ring or collar back up from the narrow end of the rod and tuck it into the wider part of the rod. This locks it in the extended position.