You really don't need a template as it is just a long rectangle, and it depends on the water bottle that you purchase to wrap (i.e. desani, ozarka, etc.) On the big side they can be 2.75x8.75 inches but for an Ozarka water bottle they can be 1.5x8.75. The best thing to do is decide what brand of water bottle you want to make a wrapper for then buy it and remove the wrapper and measure it. Make sure that you give extra room on the width as you will want to overlap the sides when you wrap it around the bottle.
I like the ozarka bottles as you can make a smaller wrapper and use less ink!
Hope that helps!