title: Define size for .gif created by gganimate - change dimension / resolution
body: <p>I'm using <code>gganimate</code> to create some .gif files that I want to insert into my reports. I'm able to save the files and view them fine, however, I find that the displayed size is small: 480x480. Is there a way to adjust that - perhaps along the lines of <code>height</code> and <code>width</code> arguments in <code>ggsave()</code>? </p>
  
  <p>I can zoom in but that impacts the quality poorly and makes it rather unreadable for my use case.</p>
  
  <p>Here's some sample code: </p>
  
  <pre><code>gplot &lt;- 
    ggplot(gapminder, 
           aes(x = gdpPercap, y = lifeExp, colour = continent, 
               size = pop, 
               frame = year)) +
      geom_point(alpha = 0.6) + 
      scale_x_log10()
  
  gganimate(gplot, "test.gif")
  </code></pre>
  
  <p>Below is the output for this code.</p>
  
  <p><a href="https://i.stack.imgur.com/j13q5.gif" rel="noreferrer"><img src="https://i.stack.imgur.com/j13q5.gif" alt="test.gif"></a></p>