[*Floater*]

For depth I pushed the ruler in as far as it would go. For height and width I took the inside emasurements at the mouth of the foot pocket as can be seen in the pics below (for size L).

I've often seen jetfins on ebay that don't have the size written on then and the seller has appeared unsure about it, so asking for measurements can helpful if you know what they should be. I used your site for that purpose.