Step 1: Upload your image as you normally would upload through Blogger.
Step 2: Select one of the four default sizes that you think is closest to the size you want to use.
Step 3: If you have been creating blog posts in the "compose" mode, switch to "HTML" mode.
Step 4: Now that you're in the HTML mode look for "height=xxx" and "width=xxx" then change those numerical values to make your image larger or smaller. Keep the ratio between height and width the same as you change the dimensions or your image will be distorted.