title: Change plot title sizes in a facet_wrap multiplot
body: <p>Can any help me change the title text size for these plots. i.e. make them larger? <img src="https://i.stack.imgur.com/n5N9q.png" alt="enter image description here"></p>
  
  <p>Script </p>
  
  <pre><code>ggplot(NMPSCMOR, aes(Length,  fill=Year)) + 
    geom_histogram(position="dodge", binwidth=60,  colour="black") + xlim(0, 600) +
    scale_fill_grey(start = 1, end = 0)+ 
  
  
    geom_vline(data=ddply(NMPSCMOR, Year~Morphology~Sector2, numcolwise(mean)), 
               mapping=aes(xintercept=Length,color=Year),  linetype=2, size=1) + 
    scale_color_grey(start=1,end=0)+
  
    xlab("Length Class") +
    ylab(expression(paste("Total Count"))) + #( ", m^2, ")", sep = 
    facet_wrap( ~ Morphology + Sector2, ncol=3, scales = "free") +
    theme(
  
      panel.grid.minor = element_blank(),               #removes minor grid lines
    panel.grid.major = element_blank())
  </code></pre>