There are many ways to change the PCB size in Altium. Two common methods are provided below:
method one:
1. Use Keep-Out Layer on the PCB page to draw the size and shape of the required board. Note that it must be a closed shape.
2. Select the enclosed boxes drawn. If you can't select them all at once, you can hold down Shift and select the lines one by one.
3. Design --- Board Shape --- Define from selected objects, so that a PCB of any complex shape can be drawn.
Method Two:
1. Design --- Board Shape --- Redefine Board Shape (shortcut keys D-S-R), for the square, draw 4 points in sequence, then right-click to exit the operation. Don't right-click until you draw the desired shape.
2. Set to automatically snap to target hotpots: right-click on the blank space-Options-Board Options, and select Snap To Object Hotpots in the lower right corner. This is easy to do thanks to the settings that capture target hotspots.