In the object editor, select the building you would like to change. Then under Art - Scaling value chang that number to change how big the model is. To change how big the pathing of the building is, go to pathing - pathing map.
And always remember if you change the size change the collision thing to or a unit could just walk through it.. so if u set his size to 10.00 be sure to set collision to 10.00 unless you want units to walk through him O.o