A standard shower curtain is 72" tall and ideally should be hung no higher than 6" from the floor.
More Info: As manufacturers sometimes make shower curtains in custom lengths, and people prefer different looks, it's easier to decide how high from the floor you want the shower curtain to hang and add this to the length of the shower curtain before permanently affixing the rod. For a dramatic look, the shower curtain can break the floor or hang slightly higher if preferred. You just want to be careful that you allow it to drape low enough that it is going to effectively do its job of keeping the water inside of the tub surround when pulled inside.
Things to Do before You Buy
Before purchasing a shower curtain rod, you should measure the length between the walls where the shower curtain rod will be installed. Most shower curtain rods fit a range of sizes and you want to be sure that your rod fits your space. Also, you must decide which finish is required. In general, the rod should match the other metal finishes in your bathroom. Finally decide if you want a curved or straight shower rod. It's also generally a good idea to purchase shower curtain hooks at the same time as you buy the rod so the finishes match.
Installing a Spring Tensioning Rod
If you have a straight rod, it is probably a spring tensioning rod. Clean the wall where the ends of the rod will touch. Then you will need to twist one end of the rod to make it longer. As you twist, continually check the rod against your opening to see if it fits. Once it is the correct length, put one end against the wall and then slide the other end into place. You want it to be a tight fit so it feels secure. Check the rod with a level to ensure straightness.
Installing a Fixed Rod
Curved rods and some straight rods have brackets that must be screwed into the wall. Based on the length of your shower curtain, measure up from the floor on both sides and drill pilot holes for your screws. Screw both the brackets into the wall and then place the rod in one bracket and swing the other end into the other bracket. Check the rod with a level to ensure that it is straight.