I no longer use Ubuntu, but I think the best solution is to put something like this in your .Rprofile file.
setHook(packageEvent("grDevices", "onLoad"),
function(...) grDevices::X11.options(width = 4, height = 4, pointsize = 8)
)
Mac :
setHook(packageEvent("grDevices", "onLoad"),
function(...) grDevices::quartz.options(dpi = 72))