Upload & Sell: On
This question has different types of implications. What do I prefer to see? Something that is about 900 px vertically (so I can fit the whole thing in my browser window on my 1680x1050 monitor at home).
However, I'm not going to personally share most pics at a size of 1350x900, because there are plenty of people that's too big for, and almost no one that'd be really too small for (sure I like seeing big pics, but I don't NEED to see big pics). Plus, since I browse this and other sites on my phone a lot, having to zoom out for the pics, then in for the text all the time can be a pain.
The really large resolutions I don't share at because I don't want people to be able to easily make decent size prints from my web images. Once you start hitting 2MP images, you can start making pretty good 5x7 and decent 8x10 prints.
My web resize macro in Photoshop resizes to fit 900 px horizontally and 750 pix vertically. This way, it'll easily display well on most monitors, and is still big enough to see decent amounts of detail.
Things in a 16:9 aspect ratio, I tend to share at 1000 to 1200 pixels wide, and wider panoramas, I will share up to 1500 pixels wide, as those look terrible at 900...you lose a lot of the detail.