When composing a new message the size of the box you are supposed to type your text in is alright, but when you answer to an email the box becomes tiny. Particularly on a widescreen laptop screen you are only given about one fifth of the screen for your response. The Gmail reply box is really way too small this way, it is almost impossible to read the email you are responding to, which forces you to permanently scroll up and down.
Simply click the diagonal arrow in the right corner of your response box.
This will open a new window, which when maximized gives your plenty of room for your response.
Alternatively you can press ALT when clicking “Reply” to open a new window:
A nice side effect is that your original Gmail tab will still be there to check back and search other emails if needed for your response. I used to open duplicate tabs, which was a bit confusing because suddenly I had two open tabs with my response. I really got used to answering emails in a new window. With F11 you can toggle full-screen mode on and off, which makes the box even bigger.