The critical dimensions are shown in AWS D1.1 Figures 4.15, 4.16 or 4.17, depending on which type of bender will be used.  The spacing dimensions for the bend radius depend on the yield strength of the steel being tested.  The most common is the 50 ksi and under steels which require a 3/4 inch bend radius.  If you are planning to build a bender and claim that it meets D1.1, you should find a copy and review the dimensions/tolerances first.  Since D1.1 is copyrighted, the figures can't be posted for public use.  You'll need to either purchase or borrow a copy.