Having just rewired my house, I feel the need to chime in. Everything said so far is absolutely correct. There is NOTHING wrong with having wire that is too big for your circuit. You MUST change your breaker to a 30A. It will WORK with a 50, but as others have stated, it will be a fire hazard. I will say, are you SURE your dryer is looking for 30A only? That seems awfully low for a dryer, unless it's a very small one. Keep in mind, a 30A 220 breaker is two 15's tied together. I've never heard of a 50A 220 circuit (in fact, I've never seen a 25A breaker.) If there is only 1 50A, then what you have is 110, not 220. Keep in mind, this is all true in the states. I have no idea which side of the pond you're on, and have no clue as to how things work in Europe. One other thing to watch out for is that since a 30A won't need the same size wire as a 50A, there's a decent chance the wire won't fit into your breaker without trimming some of the wires off first. (Wire of that gauge is usually stranded so it can be bent without tools.)

Last but not least, check the voltage on the panel with a meter to make sure it's off before you work in your panel. My electrician friend who helped me rewire almost taught his assistant this lesson the hard way at my house. We pulled the meter, the assistant starts to pull a breaker. Jim says "Did you check the power to make sure it's off?" The assistant looks at him like he's nuts, but pulls out his meter. Checks the voltage across the panel, and reads 220. Keep in mind that the power meter is sitting on the ground at our feet. Turns out that in MY house, for some unknown reason, a commercial meter pan was used, rather than residential. In a commercial pan, when you pull the lever to release the meter, there are these bars that slide into contact completing the circuit behind the meter pan. That way, the power company can do whatever they need to do to a meter without interrupting power to the business. Scary. He coulda, and I woulda, been toast. Hurting at the least.

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Of course, after 2 seconds of research, 25 and 50A breakers are quite common, I've just never seen one.