1) Go into Build/Buy mode
2) Choose the object you want to modify size, by left clicking on it once
3) On your keyboard, press Ctrl + [ (square bracket on your keyboard, this is to make it smaller)
4) You have to move the object to see the size changes.
5) If you want to make it larger, use Ctrl + ]
6) Same thing, move the object and you will see it gets bigger.
EDIT: You cannot make them smaller than original size.