Hi Angelo

This is how I'd do:
- First, in the main program--> detail tab--> create a square around the 300x300 area--> delete all the elements out of the square (walls, plants, objects, etc)
- Go to Power tools--> Topo designer--> select the 300 x 300 area --> create a building pad in this area (don't worry for the results, is temporary)--> delete all the topo elements out of the building pad--> Design--> lot properties--> 300x300--> after you set the lot in the desired dimension, you can delete this building pad as well as the square in the detail tab (main program).

Hope this helps

Patricia