Let's assume that the Heroica figures are exactly half the scale of the normal LEGO minifigure. That would be roughly 1:100. At that scale, 1 meter would be roughly 0.75 studs long or the height of a standard LEGO brick.
Now let's convert the Coliseum dimensions:
Height: 50 meters ~ 50 bricks high
Length: 189 meters ~ 142 studs
Width: 156 meters ~ 117 ...