You'd need a pretty big exercise ball, depending on the height of your desk. I considered using the one I have but I don't see it as a usable long term solution, isn't bit enough, and would look ugly in my living room.
KaelangMy desk is a dining table from ikea so it's not that tall and because it's in my bedroom, I don't care what my chair is. I like the ball more so that I CAN move and burn even the slightest more calories than normal...