Generally, as CodesInChaos notes, readability is an issue for lowercase letters at such small sizes. That said, I just tried it, and it is possible to make a fairly readable "Cr" icon at that size:

(The image above is in .png format because imgur won't let me upload an .ico file.)
Anyway, let me hereby relinquish any copyright claims I might have to this image. I suppose the original copyright belongs to Stack Exchange, so if they want to use it in any way (like, say, as the new favicon for this site... hint, hint) they're welcome to do so.
Cr
is hard to read when using such a small font. $\endgroup$