Here's an example that wraps long split labels over multiple lines. The maximum length of each line is 25 characters. Change the 25 to suit your purposes. (This example is derived from Section 6.1 in the rpart.plot vignette.)
tree <- rpart(Price/1000 ~ Mileage + Type + Country, cu.summary)
split.fun <- function(x, labs, digits, varlen, faclen)
{
# replace commas with spaces (needed for strwrap)
labs <- gsub(",", " ", labs)
for(i in 1:length(labs)) {
# split labs[i] into multiple lines
labs[i] <- paste(strwrap(labs[i], width=25), collapse="\n")
}
labs
}
prp(tree, split.fun=split.fun)