How do I change the dimensions of a PCB in Altium Designer 19.1 using by entering the value I want the dimension to be?
Create a polygon (place polygon plane) on whatever layer you like. Mechanical 1 is a good choice since it is often used by Gerber export as the board outline layer. The "Outline Vertices" shown in the object properties when it is selected will allow you to specify exact values. When done, select Design > Board Shape > Define from Selected Objects. (Tested in Altium 19.1.8)