the size parameter worked, i had missed the colon before the word size, ":size=width,height"
what is concerning is the this parameter is undocumented and worked, while the documented parameters of --width and --height don't seem to work.
Thanks for your help
Hi! For the --width and --height tags to function properly, you need to set the Dashboard size to "Automatic" in Tableau Desktop before publishing/republishing to Tableau Server.