[R] Change margin size of complex barplot

[Michael Eisenring]

Dear R-help members,
with the help of one of you the following R-Code was developed. No I have the (probably simple) problem that I want to increase font size of "text" and "ylab" names  from 1 to 2. Unfortunately I'm not able to adjust the margin so, that the plot is appers complete (all names readable) on a 8:14 Inch scale plot. 
Some times I get the error message that the margin borders are to big (if I increase the oldpar value), sometimes nothing happens , I have no clue why. Can anyone help me? 
Thank you
Michi
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R CODE
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dput(geo_barplot_percentage_top10)

geo_barplot_percentage_top10 <- structure(list(young = c(43L, 7L, 3L, 2L, 2L, 2L, 1L, 1L, 1L, 1L), old = c(21L,9L, 4L, 3L, 3L, 3L, 2L, 2L, 2L, 2L)), .Names = c("young", "old"), class = "data.frame", row.names = c(NA, -10L))

                                                                                                                                                                         
bp <- barplot(data.matrix(geo_barplot_percentage_top10),
              width = 1,
              ylim = c(0,50),
              xaxt = "n",
              space = c(0.2, 1.0),   
              ylab = "Relative frequencies (%)",
              beside = TRUE,
              col = rep(c("orange", "red"), each=10),
              cex.names = 2)
oldpar <- par(mar=c(18, 7, 2, 2.1))
oldfont <- par(font=3)

  names.arg = rep(as.character(1), 3))
text(as.vector(bp),y=-1, cex=2,pos = 2, 
     offset = -0.1,
     labels = c("Asthenotricha cf. unipecten 1","Asthenotricha cf. unipecten 2","Cleora cf. pavlitzkiae","Omphalucha cf. apira","Asthenotricha sp. 6","Prasinocyma cf. neglecta","Asthenotricha sp. 5","Scopula cf. cauducaria","Zamarada cf. melpomene","Eois cf. oressigenes","Asthenotricha cf. unipecten 1","Asthenotricha cf. unipecten 2","Asthenotricha sp. 6","Asthenotricha sp. 5","Cleora cf. pavlitzkiae","Colocleora cf. polyplanes","Miantochora cf. fletcheri 3","Miantochora cf. fletcheri 1","Pingasa cf. rufata","Xanthisthisa cf. tarsispina")
     ,srt = 50,xpd = TRUE)#srt=text rotation

legend.width <- max(strwidth(c("Young secondary forest","Old secondary forest")))
par(oldfont)   # back to normal font for legend
legend(14,62,
       c("Young secondary forest","Old secondary forest"),
       cex=1.5, pt.cex=4, pch=15, y.intersp=0.15,bty="n",
       text.width = legend.width/2.5,    
       col=c("orange","red"))
#8:14 Scale of graph!