leaflet changes labelOptions

Question

I created a Map and would like to change the textsize, so my code looks like that:

 map <- addLabelOnlyMarkers(map = map,
                           lng = x,
                           lat = y,
                           label = label,
                           labelOptions = labelOptions(noHide = TRUE,
                                                       direction = "center",
                                                       textOnly = TRUE,
                                                       opacity = 0.9,
                                                       textsize = "10px",
                                                       sticky = TRUE))

But my problem is that I want to change the textsize but if I do that I see no change, for example if I write textsize = "5px" instead of 10px. Isn't it possible with that package?