Fill an Entire PowerPoint Slide with an R Plot?

How can I fill an entire PowerPoint slide with an R plot? I'd like to use vector graphics (otherwise screenshot and crop would be a solution). I'd also like to avoid manual touches.

The following code (grabbed from this post) does a perfectly fine job of filling the "Content" of a "Title and Content" slide.

library( ReporteRs )
require( ggplot2 )
mydoc = pptx( )
mydoc = addSlide( mydoc, slide.layout = "Title and Content" )
mydoc = addTitle( mydoc, "Plot examples" )
myplot = qplot(Sepal.Length, Petal.Length
               , data = iris, color = Species
               , size = Petal.Width, alpha = I(0.7)
)
mydoc = addPlot( mydoc, function( ) print( myplot ), vector.graphic=TRUE)
writeDoc( mydoc, file = "test plot.pptx" )

I thought I might be able to fill an entire slide using the following code:

library( ReporteRs )
require( ggplot2 )
mydoc = pptx( )
mydoc = addSlide( mydoc, slide.layout = "Blank" )
#mydoc = addTitle( mydoc, "Plot examples" )
myplot = qplot(Sepal.Length, Petal.Length
               , data = iris, color = Species
               , size = Petal.Width, alpha = I(0.7)
)
mydoc = addPlot( mydoc, function( ) print( myplot ), vector.graphic=TRUE)
writeDoc( mydoc, file = "test plot.pptx" )

But, i get the error message:

Error in next_shape_pos(doc) : 
  shape of type 'plot' has no more room left to be displayed in the layout

I think there may be a solution where I create new PowerPoint slide layout with content filling the entire slide and then save as a PowerPoint template? But I'm not very familiar with this stuff and am starting to feel like I'm missing something obvious.

You have effectively answered your own question. You need to create a template in PowerPoint with a new layout that has a single content placeholder filling the slide.

In PowerPoint 2016, you would do the following:

File -> New -> Blank presentation
Delete the first (and only) slide
View -> Slide Master
Slide Master -> Insert Layout
Delete all existing placeholders on this layout
Slide Master -> Insert Placeholder -> Content
Resize the new placeholder to fill the slide
Right click on the layout in the left-hand layout list
Rename layout
"Full page content"
File -> Save as -> Browse
Choose a location and name for the presentation template (e.g. "Documents/R/BlankPresentation.pptx")

Now run the following R code:

library(ReporteRs)
library(ggplot2)
mydoc <- pptx(template = "path to presentation template saved above")
mydoc <- addSlide(mydoc, slide.layout = "Full page content")
myplot <- qplot(
  Sepal.Length,
  Petal.Length,
  data = iris,
  color = Species,
  size = Petal.Width,
  alpha = 0.7
)
mydoc <- addPlot(mydoc, function() print(myplot), vector.graphic = TRUE)
writeDoc(mydoc, file = "test plot.pptx")