Ubercart uses the ImageCache module to control the display of product images. When you configure Ubercart for image support, Ubercart creates a number of "presets" for ImageCache which define how product images are displayed in various situations.
If you go to the ImageCache settings page at admin/settings/imagecache you will find the list of all the presets along with details of how each preset displays images. The preset that controls the image display on the product page is called "product"; the "product" preset defaults to scaling the images to fit in a 100 x 100 pixel box, while preserving the original image's aspect ratio.
Expand the fieldset for the "product" preset and modify the settings to fit your needs. Be sure to submit your changes by pushing the "Update preset" button. It's also recommended to push the "Flush preset images" button to clear out any images that were built using the previous values. You might also need to flush your browser cache before you will see changes on the product page.
Note that by modifying the other presets, you can also control how images appear in product teasers, on the cart page, in the product list page, etc.