I'd like to add a cowbell to my bongos/congas/chimes/misc shakers/woodblock percussion setup.  What's a good size to start off with?

Here's an alternative to cowbell, just in case you're playing it by hand rather than stand mounted, and/or if you're interested in getting a nice bell sound with less volume. When playing in close conga circles our group has chosen the smaller prettier sounding "Atoke". We much prefer their tonal quality and ease on the ears.
Here's a description lifted from a Berkley music website:

Atoke is another instrument of the forged-iron family that produces a bell like tone when struck. It's traditional form is similar to a miniature boat. A thin forged iron rod is used to play it. Atoke is held in a partially open palm of the performer's weak hand, resting diagonally across from the fore finger to the heel. The iron rod beater is carried in the strong hand and rebounds as it strikes the rim of atoke producing a high ringing pitch. A pair of atoke, tuned about a fourth apart, always play together and function in a similar manner as gankogui.


a pair of Atoke


technique

And here's a good price:
http://www.earthshakingmusic.com/itemdetail.lasso?-database=EM_Products.fp3&-layout=web&product%20id=32994&-search

  Hey great! I'm sure you'll love it!
We just yesterday played Guiro versions of Bembe and Llesa, which consisted of super tumba and three chekere taking over the tumba, segundo, & quinto; normally played on drums. The atoke bell just blended right in pleasantly where as a cowbell would have ruined the whole thing.