It’s very difficult to know, what with variations in sizing as well as variations in size of the boot you’ll be wearing. When I upgraded from 5 mm soft sole neoprene boots to 6.5 mm semi-rigid sole neoprene boots I had to go up in size. Same thing when I upgraded to converse All-Star boots for my dry suit.
Here are some hard numbers from me, though nowhere near the size of your wife: I wear a size 12 1/2 US. In standard 5 mm wetsuit booties, I use a vintage black jet fin (the ones without the hole in the tip of the fin) in size XL. With the more shoe-like 6.5 mm boots, or the size 15 converse All-Star boots for my dry suit, I use an XXL. Those were bought new a few years ago, but are the old style jet fin, again without the hole in the tip.
As you can see from the numbers you quoted in your description and the sizes that I’m using, I am using fins one size larger than you would anticipate. However, it matters which jet fin you’re talking about. I keep harping on the fact I’m using the vintage style because the foot pocket on them is shaped noticeably different than the modern style jet fins, the ones with the hole in the tip. I love my jet fins for propelling myself through the water and balancing my trim, but I really don’t like the foot pocket on the vintage jet fins. It’s very, very tight vertically. I have unusually high arches, and combined with the low pocket of the fin, it has caused problems with pain on the top of my feet. The more rugged shoes have basically eliminated that problem, but means I had to scale up all the way to XXL. For someone who wears a 12 1/2 that seems big, but it’s what it took.
I echo
@lexvil : if you’re going to have to order these online, make sure you use a place that has easy returns. Scuba.com (used to be leisure pro) has a pretty reasonable return policy. I’m not the biggest fan of returning things with Amazon if I can help it, just because the vendor tends to get more screwed than they do when you’re dealing with the company directly. But hey, the vendor knows that’s the cost of being on Amazon, so if it works for them, I guess I shouldn’t worry about it.
But to at least minimize the number of returns, know that the general rule with jet fins is scale up one size, and make sure you understand the difference between the vintage foot pockets and the modern foot pockets and select accordingly.
And if you can, update us with your results, just so other people in the future can benefit from your choices as well!
