This is one of the weird areas where Fender fails to provide any useful information and the smaller set screws are nowhere to be found as a Fender part while the larger ones are easily obtained. Go figure.
You may be in luck:
On my 2013 Am Dlx Jazz the concentric knob sets use a standard set screw for the small upper knobs and a smaller set screw for the lower ring knob, even though that seems backwards. I'm assuming that the small concentric knobs and the normal non-stacked knobs on my bass use the standard size 8-32 x 3/16" screws. http://www.fender.com/guitar-bass-pa...-for-knobs-24/
Also, these guys should be able to help you verify this: http://www.buzzardsbass.com/
Edit to add:
Also, the replacement screws pictured on Fender's website are slotted while the screws on my bass are hex. I just dug up an unopened package of genuine Fender "Jazz Bass Knobs" and they come with 8-32 hex screws.