I've started sending characters till i reached the maximum which is 63 bytes. I was quiet surprised of that because i've read somewhere that the Arduino has a 128byte Serial buffer.
Prior to 1.0, the Arduino did have a 128 byte buffer. With 1.0, outgoing serial data is now buffered, too, so the buffer size was cut in half, so that the two buffers take up no more space than the one buffer did.