If you open a .deck file in a text editor, near the top is a line which should look something like:

size:[512,342]

This specifies the width and height of the deck in pixels, respectively. The minimum size Decker will accept is 320x240, but theoretically it will accept dimensions up to 4096x4096. Keep in mind that large deck dimensions will negatively impact performance and result in significantly larger file sizes. Very small dimensions may pose issues for some dialogs and menus.