Rendering bigger is a good idea if your machine can handle it - that includes both rendering the image in the first place, and being able to work on it in Photoshop or whatever. If you're going to do postwork, a bigger image will mean you can zoom in on small details better. When you saved your large render: did you export the image from Poser, or just screen capture it? An exported image shouldn't include the task bar, or any other part of Windows.