My cockpit coaming is 32" long, 18" wide. I bought the sprayskirt first, and made the coaming to fit it. It fits almost too good- it can be a little hard to get on. I do like that I can pull up the front lip to let the hot air out and the skirt will stay sealed everywhere else. I recommend buying a skirt first and then making your coaming.

Note: My sprayskirt is the Harmony gore-tex skirt. I really like it- it breathes very well. I did have to replace the seam tape after storing it for a while in a hot closet, but that's common where I live.