There are three sizes of the Default Start button; small, medium, and large. You will need to click the "Pick image" button on the Start button tab in order to access them. However, there is only one size for the Start11 logo button. The color of the top, left square on the Default button and the background of the Open state are determined by the Start11 menu color.
I'm not sure how to create a new button which uses the chosen Start11 menu color, but would be interested in how it's done. It's probably built into the software code.