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.