When measuring for the Green Drain Floor Drain Trap Seal you want to remove the drain grate and locate the drain outlet. Measure the outlet diameter and that is what size you want to order.
The Green Drain Floor Drain Trap Seal fits into either the inside of the floor drain body outlet connection or the throat of the strainer. You should measure the diameter of these parts to determine which size to order. The Green Drain Seal gaskets will flex up to 0.100 inches to fit into pipes. The chart on this flyer and these installation instructions have great pictures to help you determine if this seal will fit your needs.
The Green Drain Floor Drain Trap Seal comes in several