Consider the following ggplot2 graph with long facet/strip text broken in two lines. The text goes outside the area devoted to facet titles.
library(ggplot2
ggplot2 supports a built in way of doing this using label_wrap_gen.
x <- c(1:3, 1:3)
y <- c(3:1, 1:3)
grp = c(rep("group 1 with a long name",3),rep("group 2 with a long name",3))
d = data.frame(x = x, y =y, grp = grp)
ggplot(d, aes(x=x,y=y)) + geom_line() + facet_wrap(~ grp, labeller = label_wrap_gen(width=10))