specifying dimensions of a graphic (not the window...)

Greetings --

Graphics newbie (I generally don't use R for graphics, so if the 
question is 'obvious', point that out gently ;-)

I'm trying to use levelplot in the lattice package, to generate what 
I'll call a 'decision table', where optimal decisions (discrete, on the 
interval [0.0,0.5] by increments of 0.1) from a dynamic programming 
problem are plotted as a function of time since the time horizon. The 
'matrix' I'm trying to plot is 100 rows x 10 columns. While using the 
following works, more or less...

rgb.palette <- colorRampPalette(c("red", "green"), space = "rgb")
levelplot(t(results$policy), main="optimal harvest", xlab="time from 
end", ylab="state (N)", col.regions=rgb.palette(6), cuts=6, 
at=seq(0,0.5,0.1))


the rendered figure is waaaaay too narrow. I want to make the 
proportions of the 'levelplot' (what I usually call a 'heat map') square 
(or something else that I specify). I'm used to dedicated graphics 
applications wherew I simply grab the figure and resize it to whatever 
aspect ratio I want. Obviously, with R, I need to invoke some sort of 
command line argument.

In my searches, I've found a fair number of queries/answers about how to 
change the size of the graphics windows, but I could care lerss what the 
graphics window sizing is (presumably, it should adjust to whatever the 
size of the underlying graphic is). I want to 'hard code' the dimensions 
of the graphic itself, not the window it's rendered in.

I'm sure the answer is out there, but I've been unsuccessful at finding 
the magic keywords in my searches.

Thanks in advance.