[NTG-context] Getting page width and height in Lua

If I want to get the paper width and height to use in some Lua code, I can use: local paperwidth, paperheight = backends.pdf.codeinjections.getpagedimensions() But what if I want to get the page width and height? That is, the width and height of the first argument in the two-argument version of \setuppapersize. Does anyone know how I can obtain these using Lua? Many thanks, Jack