We had this discussion before.
http://www.quiltingboard.com/vintage...m-t262798.html
I don't have an older Pfaff free arm, so can't measure one of those.
first measurement is circumference
Singer 222 7.25" - width just under 2"
Viking 21A 8.75" - width 2.5"
Bernina 530 9" - width 2.5"
Viking 6570 9.5" (didn't dig out, but pretty sure it's the same - 2.5", I know it's narrower than the 990)
Viking 990 10" - width 3"
Pfaff 2140 10.25" (this is the newest of the group) - I know this one is also the widest.