Lem:
Fins and masks are very personal items of gear.
There is a German Standard - DIN 7876 - for fin sizing, but it's not universally applied, so there's no telling whether an individual model of fin will fit you widthwise even if it's your size lengthwise. The only way to find out is to try them on, wearing booties if you intend using them for protection from rough terrain underfoot. As for fin type, I once went snorkelling at La Jolla Cove, a great snorkelling venue in Southern California. The snorkellers there happily and successfully wore every kind of fin: open-heel surfing fins with fixed straps, adjustable open-heel fins and full-foot fins with regular and long blades. The beauty of snorkelling is that "anything goes" when it comes to equipment. Scuba divers will tell you that you need big clunky open-heel fins, while freedivers will tell you that long-bladed carbonfibre-reinforced fins are necessary, but when they say this, scuba divers are really talking about scuba diving and freedivers are really talking about freediving. The important thing is that you feel comfortable with the fins you eventually choose and only you can determine that, not somebody else just speaking from their own experience and circumstances. If your fins are too loose and you are wearing them barefoot, you'll get blisters or the fins will fall off in a current. Too tight and you'll get cramp. Try them on.
As for masks, they will leak unless you find one with a skirt that seals against your particular face dimensions. When you try one on, press it against your face without using the strap, breathe in to create a vacuum and see whether the mask stays in place. If it doesn't it will leak. Submitting your future mask to this simple test is essential.
Finally, do remember that you will inevitably find fault with whatever you choose as your beginner's equipment. When you get to your snorkelling location, have a look at what your companions are wearing in the way of gear. Ask them why they made the gear choices they did. Check out the snorkelling equipment stores at your destination. Japanese manufacturer Gull makes some great snorkelling fins that are sold in the Far East, including the Philippines, but not in western countries. All this will be a valuable lesson if or when you decide to purchase better equipment for your next snorkelling trip.