How is viewport size typically passed as an argument?

I'm looking at the docs for a command line tool, and in the section indicating options there is:

--viewport-size <> Set viewport size if you have custom scrollbars or css attribute overflow to emulate window size

Just wondering how this viewport-size is supposed to be passed. Just width in pixels? Two arguments for width and height of the viewport in pixels? I'm experimenting with it but the results are somewhat confusing. For reference here is the documentation page.

Thanks!