mike wrote:

[...]
My question now is, how do I measure out for a skirt? All the skirts I
have seen online give a single number for the size. What makes up this
number?
[...]

Here in Ol' Germany, I would cut a sheet of paper in the shape of my
kayak seat porthole and take it to a lokal dealer who does produce kayak
skirts (http://www.zoelzer.de). The result would be a perfectly fitting
skirt for my kayak, no matter the brand.

Rgs
Alf