First off, height doesn't matter except in rare circumstances, so I say the height should be whatever you feel like. As for the width: About 600 pixels wide is fairly common and fits well with most webpage designs, forum posts, deviantart's defaults, drunkduck, etc. You can go to about 650 with decent readability on just about any device and any website. Others go with 800-900 pixels-wide, which is great for anyone with a decent monitor, but a little tough on mobile gadgets. It also may cause you to run into resizing issues on drunkduck and in forum posts... but if you're uploading strictly to your own website, it shouldn't be a problem.