The goal of choosing the correct size is to make sure there is enough support to hold the lens in place, without making it so large that removal and replacement are difficult.
The most reliable measurement is of a currently installed panel as long as it fits securely in the frame and is not too tight.
Our lenses are sturdy enough to remain in place with as little as 1/4 inch support from the frame along the perimeter; however, optimum overlap on the frame should be 3/8 inch to 1/2 inch around the perimeter. This means the lens will be 3/4 to 1 inch longer than the length of the light opening, and 3/4 to 1 inch wider than the width of the light opening. Where more vertical space is available in the fixture, you may have up to 3/4 inch of perimeter support. Keep in mind that you need a bit of vertical clearance to be able to maneuver the lens into the opening.
If you require a custom size, our graphic artists modify the artwork to fit your specific measurements. We never simply cut part of the design off of the panel.