For a giant construct, 3-5+ seems like a reasonable ball-park figure. It depends on how massive and powerful the Construct is as well as how powerful you as a Keeper is (you can't make something more powerful or as powerful as yourself, that would be ridiculous). There is also the logistical consideration of how to care for and house your giant. Most things have to eat, and with giants this is more-so. At the very least you will need a much bigger entrance.