Rather than measuring, which would be difficult with a broken pivot, here's a quick rule of thumb:
If the serial number is under 3,000,000, you probably need the old style(short) staff. If the serial number is over 3,000,000, you probably need the new style(long) staff.
Short vs. long hub is easiest to determine by comparing the pictures of your current staff with the pictures available in the material catalog(which is available online). The difference between the two is pretty obvious even without magnification.
Chances are pretty good that you need a new style, long hub staff. This is the most common type, and in fact will commonly just be called simply an 857 when ordering(the others will specify O.S. for old style and S.H. for short hub).