I have used Altium designer 9. But i think it will be same in altium 10.
You can see the option "system". click it and open "files". You can see the option "New from template". In that, Click "PCB Board wizard". It will pop up another window, there you can mention the board size on the 'choose board details' and all other details for the board.
1) open one pcb file
2)set reference
3)place line (draw required length on any mechanical layer)
4)select all lines
5)go to design/board shape/ define from selected objects