Thanks everyone. Is there such a thing as "sizes", just like clothing (S, M, L, XL)? Or is it only to try everything that looks good?
Neither masks nor snorkels come in different sizes, but that doesn't mean they are all the same size. Some masks are designed for narrow faces, others for broad faces, although they are rarely labelled as such.
As for fins, they are indeed sized, but in size ranges, e.g. 10-12, rather than single shoe sizes. This means that somebody with size 10 feet may find 10-12 sized fins a little loose, while people with size 12 feet may find the same pair a little on the tight size. Be aware that there is more than one kind of sizing when it comes to fins:
XXS - XXL: Pretty meaningless system. XXL can be anything from US size 12 to US size 19.
US sizes: Child 1-13, Adult 1-19
UK/Australian sizes: same range as US, but the number tends to be one less than the corresponding US size.
21-52: European sizes
The following table might help:
http://www.size-chart-shoe-clothing...uploads/1/2/9/5/12950690/1819372_orig.jpg?611
A further complication is that some fin model foot pockets are not "true to size", running larger or smaller than indicated. So my advice remains: try everything on, don't rely on the sizes embossed on fins!