How To Specify Max Image Width And Hieght In Imacros Script For SAVEAS Command

Question

I am currently using this script for taking screenshots on Firefox

VERSION BUILD=7210419 RECORDER=FX
TAB T=1
TAG POS=1 TYPE=TITLE ATTR=* EXTRACT=TXT
SET !VAR1 {{!EXTRACT}}
SAVEAS TYPE=PNG FOLDER=* FILE={{!VAR1}}.png

Now, my question is, how do I specify a particular size of image to save?

I have checked out Save As Command on wiki page of imacros but I can't see anything there...