So!
I did get a merfin for christmas! just thought I'd update you on that. The foot pockets are very tight! but roomy on the inside where your toes fit.
I did have to cut the openings for my feet to get in and that makes it easier and easier still if it's wet first!
as for the length. the pockets very rounded at the end so there was no room for my big toe (stupid toe!) for the best fit I recommend grabbing your tape measure and measuring from your heel to your middle toe. If this is 24.2cm and your big toe is longer that this you aren't going to fit.
I had to cut mine so that my big feet have that extra room. while it's not attractive it does the job, and I figure that once I have a completed tail (however long that will take) you won't see it anyway.
The fin does come with a card attached that says that you would be better off wearing some neoprene or something or other socks with the fin to stop it from rubbing.
It's a bit heavier than I was expecting (it doesn't weigh a ton or anything, I'm just used to the weight to 2 separate fins rather than one single one)

I haven't gone swimming in it yet as i'm out of money and haven't been able to go to the pool. but I have sat around with it on, as well as filling up the bath to see how it feels in water.
I got it in the aquamarine and its very pretty.