Here is a video explaining the stock images on the Cricut Imagine that are indicated with a ratio. I.E. the 1:2, 2:3, 3:5, 5:7, 8:10, and 8.5:11. These can be scaled by changing the size of the rectangle and result in a variety of rectangle sizes.
You may not be able to get the exact size you want but you can get close. Some trimming may need to be done. As you can see from the chart below, not everything is going to scale to a nice round number.
Watch the video on how to make these changes and reference the chart below for the math already done for you.
Print it out and keep it by your Cricut Imagine for quick reference.
Hope this helps you guys out! Scroll down to see the table.
Chart for the proportional rectangles on the stock images: