You can use the layouts package, as discussed in another answer, to list and depict memoir page sizes, beyond the standard ones on offer.
You will have to manually tune and adjust the layout, to properly typeset a 6" x 9" document, suitable for publication in (publisher-dependent) US Trade Book format.
Guidelines for this process can be found within Section 4, of the layouts User Manual, entitled "The memoir class page layout".
Here is some sample code, adapted from there, to help you get started:
\begin{figure}
\currentstock
\trystockheight{10in}
\trystockwidth{7in}
\trytrimtop{1in}
\trytrimedge{1in}
\trypaperheight{9in}
\trypaperwidth{6in}
\tryspinemargin{0.75in}
\tryuppermargin{1.0in}
\tryheadheight{12pt}
\tryheadsep{24pt}
\trytextwidth{5in}
\trytextheight{7in}
\tryfootskip{24pt}
\setfootbox{50pt}{50pt}
\trymarginparsep{17pt}
\trymarginparwidth{62pt}
\trymarginparpush{12pt}
\setlabelfont{\small}
\stockdesign
\caption{An experimental US Trade Book \file{memoir} class page layout}\label{fig:pmemudf}
\end{figure}
You can, however, directly use geometry with memoir. So you could just try adding something like the below to your preamble:
\usepackage[paperwidth=6in, paperheight=9in, bindingoffset=0.75in]{geometry}
As mentioned in the previously-linked answer, however, it might be better to do this directly with memoir commands, as discussed above.
I also suggest you read this as well as these guidelines, to improve your experience on TeX.SE.
– Coby Viner Aug 13 '22 at 04:29