You can do that with this order:

plot(as.dendrogram(hc_sbd_1), nodePar = list(lab.cex = 0.5, pch = NA))

If you also need to make the lines thiner, use this order:

plot(as.dendrogram(hc_sbd_1), nodePa r= list(lab.cex = 0.25, pch = NA), edgePar = list(lwd = 0.5))

I you also want to change the y axis, use these orders:

plot(as.dendrogram(hc_sbd_1), nodePar = list(lab.cex = 0.25, pch = NA), edgePar = list(lwd = 0.5), axes = F)

axis(2, lwd = 0.3, cex.axis = 0.3)

And, finally, if you have a very large number of elements in the dendrogram, use these orders (they wirthe the dendrogram as a pdf file; you can change it to EMF or other output you want):

par(cex = 0.5)

pdf("hc_sbd_1.pdf", width = 20, height = 20)

plot(as.dendrogram(hc_sbd_1), nodePar = list(lab.cex = 0.05, pch = NA), edgePar = list(lwd = 0.5), axes = F)

axis(2, lwd = 0.3, cex.axis = 0.3)

dev.off()

You can adapt these orders according to the size of the dendrogram. lab.cex always refer to the size of the characters and lwd the width of the dendrogram lines.

par(cex = 1)