If you are a 1.8.1 user and choose this free "upgrade" to 2.0 Express, you actually lose the ability to set custom sizes for your exports. Those features are now reserved for the paid Pro version. Given that Express is pretty much like 1.8.1 minus the OS 10.4 look (look, ma, no brushed metal), you may want to preserve your options and secure a copy of 1.8.1 before you try or update to 2.0 Express.