You choose the proper size panel decal based on the diameter of your downtube. Panel decals wrap around the entire downtube. Most of time they are very close to touching, and sometimes they overlap a bit at the bottom.

The decal itself looks like this:

Mounted on the tube it looks like this:

Panel decals are arranged by size, from XS to XL. The width of the decal is listed next to the size. The recommended downtube diameter is listed next to the width. If you don’t know the diameter of your downtube, you can simply use a flexible ruler and measure the circumference. From there you can determine the panel that will best match your frame. Your decal choice should be the measurement that is closest to the circumference you just measured.