[R] ggplot facet label font size

[Walker, Sam]

How do I change the font size in the facet labels along the edges of the
plot?

For example (from the ggplot help file):
     p<-ggplot(tips, sex ~ smoker, aesthetics=list(x=tip/total_bill))
     gghistogram(p)

In this plot, the facet labels are "smoker: No", "smoker: Yes", "sex:
Female", "sex: Male".  What command can I use to reduce the font size of
these labels?

In lattice terminology, cex is used to scale these strip labels.  But I
couldn't find the equivalent in ggplot.

The reason I'm asking is I have a 9x7 array of plots which I've been
plotting with lattice.  I wanted to use ggplot because I like having the
labels on the edge of the plots, but the label font size is too large
and exceeding the size of the label box.

Thanks in advance...
-Sam