Question about post composition window

I'll admit up front that I know little about website design, but I often find myself frustrated with the small windows which most (if not all) websites offer for post composition.

I was wondering why this window could not have an option to be larger in size (which would obviously make composing a post much easier).

For example, my monitor is a 24" widescreen running at 1920x1200 resolution. This equates to roughly 250 square inches of screen. The reply window I am typing in now is 9 square inches, and the U2U composition window is closer to 4 square inches in size. Why so small? Is there not some way to code an option for increasing the size of the field?

I assume that the reason for this restraint is to accomodate users running at low resolutions, but why is there no option for people running high resolutions to resize the window accordingly?

It seems to me that this could be easily addressed, though I have yet to see one site with a large post composition window.

I was just wondering if anyone could explain why this is?