That's a very newbie question but I don't remember how to change the board width and length. I changed it already but can find it anymore and I wasn't able to find it in the doc !?!
In the Design Toolbox, select the Layers tab, then select the Board Outline layer (double click, it should be highlighted in red). You can now select the board outline and either drag the points on the workspace, or you can double click the board outline and edit the width and height directly.