I have to admit that my Echo is an old narrow slalom board, 58 cm wide if I well remember, so a smaller weed fin than on you board could be the right choice for me. I've tested the Fanatic weed on the custom small-flapper board that I use outside Venice (yes, I have double quiver, this is living in Venice downtown...), 73 cm wide, and it seems not so bad, but the old flapper tail design interferes with weed fins and spinout happens some times, so not a long test.
About checking the lenght-depht, yes really Select website is not so clear, but I've seen that for almost every weed fin factory are not so clear the two sizes, mine Fanatic is the only one with clear written both numbers. Even looking on ebay.de is not always clear what you're gonna buy, f.e. a 45 cm deep or long weed fin.
And therefore I've seen that is often suggested a longer weed fin compared to the normal one, so benefits of weed fins in shallow waters are partially lost.
The famous Lessacher http://www.designlessacher.de/
talk about depht-draught and surface-area, nothing about lenght; but note that for sails over 8,0 a 36 deep fin is suggested, no matter the width of board pheraphs because of increasing of total fin surface. Shape is pheraphs better than "drop" shape of Select. Lessacher fins can be found on ebay.de in many lenght. I like them so much, and I've read very good tests on the web.
For checking the Select sizes an idea could be to print the image and then make a proportion with your actual fins, as obviously fin head are a constant size.
Hope can help and sorry for language mistakes.