Set PCB size with values altium
You can define the board size from a primitive, for example, draw the edges of the board using lines, select them all, then go to Desing->Board Shape->Define from selected objects:



Another way is using the PCB Board Wizard. You need the File menu on your side panel:





Then use the PCB Board Wizard where you can define the exact board size along with other parameters. If you don't have the File menu you can show it going to View->Workspace Panels->System->Files.