One question...

Theme: Line, drawing

How can I adapt the board outline? (also: Change board size)
The PCB outline is a closed chain of line segments being placed on layer 23, Outline (by default). Using the outline assistant you can create a basic figure. You also might construct it from single lines manually.

In order to change an existing outline afterwards please click M1 on a straight line segment of the outline. The line gets highlighted and three little black touchpads (vertices) appear, two at the end and one in the middle.

Click M2 on a pad to open a context menu. Use the menu items to add a vertex or to round a corner. Further options can be selected from the context menu.