You will hear many opinions. But the fact is everyone's legs, muscles, ankles, etc. are different.
So everyone has his "optimal" set of fins, with different geometry, stiffness, etc. The fact that for someone a particular brand/model are very effective, does not mean that they will fit to YOU.
When I was working as an instructor of finned swimming (many years ago) at the first lesson of the course we had several dozens of fins of various shape, brands and materials, and I invited each student to test ALL of them. It is time consuming, as you need to swim at least 5 or 10 minutes which each set.
But in a few hours, every student did reach a rough idea of what they need. The three main variables are length, stiffness and angle between the blade and base of your feet.
The latter is a variable which is due to change in time: the more you swim with fins, the more your ankles will become "loose", and hence the angle between blade and feet base must be reduced. Athletes doing fin swimming competition often require a NEGATIVE angle, which means that the blades "come up" from the floor while standing. Instead beginners with little mobility in their ankles usually benefit by fins with a significant positive angle.
Length and stiffness are partially interrelated, as the product of (length x stiffness) gives the power (in W) which you can release to water. And of course the max power is limited by your muscles and their training, So, again, for beginners with no training the max power is limited, hence the fins cannot be simultaneously very long and very stiff, as the "engine" pushing them is too weak.
After months (or years) of training your legs will become stronger, so you can output more power, and hence you can use longer and/or stiffer blades.
Finally, for a given power product, you can trade off between short and stiff fins towards long and soft ones. This corresponds to the gear ratio of your bicycle, and how you want to swim. For short bursts, you usually prefer a short gear ratio, with short and stiff fins. They are indeed very fatiguing in the long run. For hours-long swimming on long distances, usually athletes employ very long and much softer fins, which are less fatiguing and do not cause cramps. However, while kicking with short and rigid fins is easy for most people, driving properly in a nice sinusoidal wave the long carbon-fiber fins used by athletes require a significant training, as the motion of the legs must follows perfectly the optimal sinusoidal fluctuation of the fin blade. It take MONTHS to learn how to kick properly with these long, flexible fins. Which was mostly my job when working as an instructor of finned swimming.
All that said, you are a beginner, so I warn you to not choose fins which are simultaneously very long and very stiff. You will come there later, perhaps in a couple of years. And if you do not have a previous experience as a free diver or a finned swimmer, I would suggest to stay away, from now, form those long (and very efficient) free diving fins. Start with shorter ones, You will feel the need to change them when your muscles will grow strong enough.
Finally, be prepared to change your fins at least twice in the following months, before you reach a stable condition and a perfect matching between your legs and your fins.
