You can add the geometry package into the document preamble with optional arguments that indicate the pagesize. You can do this by choosing Typeset, Preamble, and adding

\usepackage[paperwidth=16cm, paperheight=24cm]{geometry}

If you have already added the geometry package, you can change the optional parameters by choosing Typeset, Options and Packages, Package Options, selecting "geometry" from the list of packages, clicking Modify, clicking Go Native, and adding

,paperwidth=16cm, paperheight=24cm

to the list of currently selected options.