I would suggest two things. First, use the print command instead of saveas because it gives you more control. Second, use the PaperPosition property instead of Position to control how big the exported graphic is.
Here's a quick example of exporting a 1280-by-1024 PNG-file:
% Draw your scene
plot(1:10);
% Control the output size using 'PaperPosition' -- note the 200 here... that will show up again as our output resolution.
set(gcf, 'PaperUnits', 'inches', 'PaperPosition', [0 0 1280 1024]/200);
% Note the -r200 -- we're telling it to export at 200 pixels-per-inch
print -dpng -r200 MyGraph.png
I tried the same thing with the -dmeta option (for generating an EMF-file), but it, err, didn't create the right size file. I think that might be because EMF -- since it can include both vector and bitmap content -- doesn't honor the pixels-per-inch setting the same way as a purely bitmap format.
