Apologies if this has been mentioned previously, but I couldn't find it.
For personal messages, the scrollable text box only displays five lines. This is on the system that I'm using now (OS X, Safari) and I appreciate that it may vary. The small reply box is infuriating when you are composing a response of any length. On this box, the "Reply to" box to a forum thread displays ten lines which approaches usability but more lines would be useful.