It all depends on the bottle size and shape. The only true limitation you have is the size of the straight wall of the bottle (if a straight piece of paper encounters a curved surface it creates a wrinkle), everything else is personal preference. Put a ruler to the side of the bottle and measure how tall the straight portion of the bottle is (where the ruler's edge makes perfect contact with the glass). This defines your height limitation. Then depending on your design and personal preference, you need to decide if you want it to wrap around the back or be completely visible. If you're using a standard tall desert bottle, 3" x 5" works well. If it's the shorter Bordeaux shape, then 2.6" x 3.5" works well.