On the menu bar on the top:
2DPatternWindow --> Scale Up/Down Development by Specifying Value...
Brings up the window:
Assembled Size (Height) in CM. Skip saved it at 27 cm tall, so if you wanted 25% smaller you'd just calclulate and then enter 20.25 there. You might just want to measure your son first though and see how 20.25 cm tall would look, that seems a little small.