John, unfortunately the storage lot where we keep our coach closes at 9 PM, so I can't go measure our cushions for you. However, maybe I can help you guess how big they need to be.
First, the easy part. Measure how long the cushions need to be by measuring the plywood base's length. Now you have that dimension, measure how wide the plywood base is, with it NOT extended. That will be the width of your bottom cushion. Next, extend the base out, and measure the difference from the retracted position. That will be the width of your side, or back, cushion.
Sorry I can't give exact figures, hopefully you will be able to do what is needed by following the above steps.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.