How do I adjust the size of group labels when calling plotCounts()?

I've tried passing cex.axis=.5, this shrinks only the y axis.
I've tried passing cex.lab=.5, this shrinks only the titles of the axes.
I've tried passing cex.names, as if it were a box plot.  It doesn't work at all.

I’d recommend using returnData=TRUE and building the plot from scratch. See example in the workflow:

http://bioconductor.org/help/workflows/rnaseqGene/#counts-plot