Someone posted a video that purports to be a 5 million brick LEGO boulder rolling down the hill.
Somehow I doubt that it is made of that many bricks.
A 2x4 brick measures 32mm x 16mm x 9.6mm and weighs 2.5 grams. Add it all up and we should be closer to 12 feet in diameter and weigh over 27,000 pounds.
However, if they are talking LEGO units (aka a 1x1 brick) then it might be possible. A 6 foot sphere would take about 625,000 2x4 bricks and still weigh over 3400 pounds.
I'll call bull.