I think they were 4.75 x 16. The old size is 4.5 and is not available anymore only if you want the Bombardier ones, but they do not have any tread, so I got these from Coker Tire. They have a good website.
The 4.75 will fit your B 12 with no problem. I had to space out the bars on the B 7 so they would fit. They are just 1/4" wider but fit the cleats fine.