Here is how to measure your pup's neck
Small Flower - 2" diameter (Recommended for neck sizes up to 11.5")
Medium Flower - 2.5" diameter (Recommended for neck sizes 12" to 15.5")
Large Flower - 3" diameter (Recommended for neck sizes 16" to 19.5")
XLarge Flower - 3.5" diameter (Recommended for neck sizes 20" and up)