I think they were about 4-5 inches, I didn't see the start of the incident, only heard the loud snap as they joined and self-destructed! Let's say my brother was suitably impressed with the carnage. I also know a guy that had his finger broken when he had similar magnets, obviously from less than careful handling as well.
I have slid mine off a metal toolbox and had them get together (without the carnage) once, and I had to use a vice and a large pair of pump pliers to get them apart. One of them did get a big chip in the end, but I still use it, it's on my toolbox to magnetize tools. The ones I have are the 100# variety, not to be trifled with. 
I use a VCR tape eraser to demagnetize tools if necessary, so I have both ends covered. 
