How to get size of PCB in Proteus
There's a mode called Dimension Mode. You can access this mode by clicking the button with a diagonal two-headed arrow symbol from the toolbar on the left: 



At this mode, if you draw a line, it will show the length:



If you want to see the length in metric unit instead of imperial unit, go to View menu then click "Metric". All the dimensions will show in milimeters.