Hi Pawel
Can't help with the C# issue then, I'm afraid.
If the extra 255 bytes are only at the start, a workaround would be to add a "handshake" between the PC and the Arduino. Get your C# program to send a different character, say 'B', when it starts. In your sketch, read serial one byte at a time, discarding bytes until you see the 'B'. To be completely sure, you could then have the Arduino send back a specific character and have C# wait until it sees this before it starts sending your proper data.
Good luck with it.
Ray