It's machine_head_with_fans_polygon.
In Cura 15.04.6 it had the following numbers:
Head size towards X min 75mm
Head size towards Y min 18mm
Head size towards X max 18mm
Head size towards Y max 35mm
However, all of them except X max seem to be very optimistic, I'd add at least 5mm to each.
In ultimaker_original.json machine_head_with_fans_polygon I can see 8 numbers, grouped in pairs:
-75, 35
-75, -18
18, 35
18, -18
So the numbers seem to be the same. I don't know if it crashed into the part because the numbers are too optimistic, or if there is a bug somewhere.
Anyway, I think there should be a way to set the printer head size without having to edit a .json file. Even if it comes with correct defaults, many ppl have modified their print heads with other fans, etc.