What size is the tiny ball bearing used in SRAM rim-brake caliper quick release detents?
The balls are 2mm in diameter.

How do I know?  Well, I found a bearing supply shop, and asked them to see some of their smallest balls.  I bought one 2mm ball and one 3/32" (2.38mm) ball for all of 20 cents.  The 3/32" ball was slightly too large to fit in the hole, so I tried the 2mm one, and bingo--a perfect fit.  Putting the QR mechanism back together is a pain because the tiny 2mm spring wants to eject the ball again, but with the brake facing straight down it's doable.