After starting the map editor, you will be presented with a blank screen full of little green dots. Along the bottom of the screen are four buttons: tiles, lights, objects and starts (starting locations). These change the current editing mode.
In the top-right corner of the screen are another four buttons, with a palette below them. The up and down buttons scrolls the current list of things displayed in the palette. The left and right arrow buttons select other lists, if any exist.
Maps cannot be resized at the moment.