Court, you need to take the parts diagrams with a grain of salt. The parts picture that pulls up for me also shows a solid disk. The actual part is vented. What part number did Guido quote you?
Anyway. I measured the opening of my calipers, though somewhat crudely, since I was too lazy to take the wheel off. The mouth is about 75mm, maybe a little less.
You mentioned that you had to push the pistons way back. The outside pistons aren't way back, rather they are near their extension limit as shown in the picture. While we are looking at the picture, that bolt in the center has no business being there. There is a specialized spring pin that crosses the chasm and holds the flat spring (not seen in the picture) in place.
Before you do push the pistons all the way back in, check the fluid reservoir to be sure it doesn't overflow.