Vinry_.: That was not until I continued reading and realized that you couldn't even see the Cancel and Post my message buttons. So nope, I think this is an issue that few people experience since I've never encountered such a problem.
It's a problem that exists on GoG's code. HTML describes how a page should be formatted, not it's size, so it will auto-scale to try to best fit based on width of data. Yes this does mean each column in a grid may be different sizes as the minimum to fit the elements and can look ugly to the eye, but as long as all the images and buttons fit the rest of the message box should scale to size.
This means the size of the reply box HAS to have a set size, either in the CSS or in the page.
Curious using 'Inspect' immediately i see that the box is set to 110 columns and 20 rows. That would be the fixed size. If taken away (
via grease-monkey or something) then it would probably scale.