You can access the Media Settings by going to "Settings" -> "Media" on the left-hand menu.
Available Options
Thumbnail size: Set the thumbnail size to be used by Wordpress when an image is uploaded. There will be a version of the image generated to this size when the image is uploaded, and it is commonly used on blog pages.
You can also set if the thumbnail should be the exact dimensions, or whether it should be proportional.
Medium size: Set the medium size to be used by Wordpress when an image is uploaded. There will be a version of the image generated to this size when the image is uploaded.
Large size: Set the large size to be used by Wordpress when an image is uploaded. There will be a version of the image generated to this size when the image is uploaded.
IMPORTANT NOTE
These image sizes are only for images uploaded to Wordpress via the Media Library, and have no effect on Category or Product images.