Remove all rows where length of string is more than n

Try this:

m[!nchar(as.character(m$f_name)) > 3, ]

To reword your question slightly, you want to retain rows where entries in f_name have length of 3 or less. So how about:

subset(m, nchar(as.character(f_name)) <= 3)

For those looking for a tidyverse approach, you can use dplyr::filter:

m %>% dplyr::filter(nchar(f_name) > 3)