A comfortable fit is indeed the primary criterion when selecting fins. Feet are three-dimensional entities. They not only have length, but also width and height. Fins are sized in a range of foot lengths. A pair marked "10-12" will probably serve a size 11 foot well but may be too loose for size 10 and too tight for size 12 feet. Too loose and the fins may generate blisters through abrasion or even drop off. Too tight and the fins may cause cramp. Then there is the matter of width. No single fin model comes in a range of widths, so certain models may have foot pockets that are too narrow or broad for your feet. There is a German Standard for fin manufacturers covering foot length versus foot width (DIN 7876) but few companies comply with its specifications. Then there is the question of the height of the arch. Here again, certain fins will prove too tight, others too loose.
There is no substitute for trying on fins at the time of purchase and even better would be the opportunity to try out fins for fit in the water. Finding an exact fit when it comes to fins is something of personal odyssey that may take several years to complete. I've found about half a dozen fins in five decades of snorkelling that are my exact fit and I rotate their use. When you find a pair that is a precise match, you will know, in the same way as you have undoubtedly discovered when locating a pair of comfortable shoes.
As for open-heel versus closed-heel, it's still as it always has been, a matter of personal preference. Full-foot fins were the norm for scuba diving back in the 1960s and 1970s, especially in Europe. Their whole foot coverage was particularly appreciated then on the pebbly shores of the Mediterranean. I always snorkel in the cold waters of the North Sea wearing a pair of all-rubber full-foot fins because they're comfortable, cover my whole foot including the heel, and suit the relaxed style of swimming I enjoy now I'm in my 60s. They're the kind of fins I started with back in the mid 1960s. Some of the more robust closed-heel fins, e.g. Apollo Bio Uni split fins and Gull Mew fins, are designed to be worn with booties, so you are doubly protected when you wear them:
Note too the "fin grips" in the image above, which enable you to wear full-foot fins that are slightly too big.
Fins, like masks, are very personal items of gear. Take your time when selecting them. Beg and borrow several pairs from fellow divers to see what suits you best. All divers and snorkellers will advocate and justify their own choice of fins, but what they have chosen will match their own physical and ergonomic characteristics, not yours, so I'm certainly not recommending that you follow my example of all-rubber full-foot fins.