EMPLOYEE0Hello Johan, You can use whatever size you'd like. I think the bigger question is how big the file size grows and if that leads to a potentially unpleasant viewing experience for some of their videos. Screencast.com will take your video as is, and not re-encode it. We will show a smaller size in the browser window so it fits, but the viewer can click the Full Size button to view it at 1440 x 1080. They will see scroll bars if their screen isn't big enough to accomodate it.
Let us know if you have more questions and what you think of the experience!