The dimensions of the button popup in the published course are fixed; a scroll bar displays if the content exceeds the available space. Author-it scales large images to fit within the popup borders, while small images retain their original size.
The image size per button is 300 x 210 pixels (width x height). If you use an image that is smaller than this size, the publishing process scales it up to the 300 pixel width, with aspect ratio maintained. If the image is larger, the publishing process scales the image down to the maximum width.
Headings for button images do not have a limited character count.