What would you want the variables to be used for? It looks like the variables you've put are to be used in plotting but its rare that you'd need users to be able to access them individually and couldn't just have them as the default arguments to a function(s). The 'data' folder is typically for datasets and tables that are needed for examples.
In general it is very uncommon that you'd need to do it, but if you really wanted to, one method you could consider is just defining helper functions:
get_major_grid_size <- function() {0.5}
# or
get_grid_size <- function() {list(major = 0.5, minor = 0.25)}
#or
get_pkg_constants <- function() {list(grid.major.size = 0.5,
grid.minor.size = 0.25,
...) #etc