The primary criterion when testing fins for one's own use is fit. Individual fin models are designed to accommodate a range of sizes, e.g. 8-10. Such a size range can mean that somebody with an 8 shoe size will find the fins on the big side, while somebody with shoes size 10 may find the foot pocket a little small. This sizing only takes foot length into account. Feet are of course three-dimensional, so the breadth of the sole and the height of the arch may be at odds with the length of the foot. Fins that are too tight will restrict circulation and generate cramping. Too loose and there's a danger of fins dropping off or at least raising blisters.
Like Malaysia and Russia, Germany has a National Standard for fins, DIN 7876 of October 1980. The bulk of the Standard is devoted to foot pocket sizing. The document stipulates which foot breadth should go with which foot length. Even so, the Standard allows a lot of leeway. And most fin makers ignore the Standard. So you would be wise to focus on getting a good fit for your own feet when choosing fins at the dive store. Finding a well-fitting pair of fins is something an odyssey. During half a century of snorkelling, I've located possibly half a dozen fin models that accommodate my feet comfortably. If you instantly find a pair that matches your feet dimensions exactly, you will have been very fortunate indeed. In all events, don't settle on a model or even a make of fins before you've solved the fit problem. Others may urge you to rush into purchasing a particular brand that suits them. Their feet are different from yours. If you intend wearing socks, booties or boots with the fins, put them on before you try on your fins for size.
Only when you've more or less sorted out several pairs that fit should you apply other testing criteria. Fins are worn for power, manoeuvrability or endurance, but not for speed. Few fins if any combine all three criteria in equal proportion, so be wary of claims about "universal" fins. It will take a while to discover which fins not only fit you precisely but also match your kicking style, stamina and the type of diving you are planning to do. It's not a process that can be rushed. Take your time.
