\documentclass[showtrims]{memoir}


% Set stock size to A4
\setstocksize{297mm}{210mm}
% Set page size to B5
\settrimmedsize{250mm}{176mm}{*}
\isopage
\checkandfixthelayout

\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing}

% Special current page bounding box rectangle that adapts to stock paper sizes
% and recto/verso pages
\newcommand\setpagenode{
    % The original current page node code can be found in the file
    % generic/pgf/modules/pgfmoduleshapes.code.tex
    \expandafter\def\csname pgf@sh@ns@current page\endcsname{rectangle}
    % Use a few low level Memoir macros to check if we are on an even or odd page.
    \strictpagecheck
    \checkoddpage
    \ifoddpage%
        \expandafter\def\csname pgf@sh@np@current page\endcsname{%
          % Set the current page.south west coordinate
          \def\southwest{\pgfpoint{\stockwidth-\paperwidth-\trimedge}%
                                  {\stockheight-\trimtop-\paperheight}}%
          % Set the current page.north east coordinate
          \def\northeast{\pgfpoint{\stockwidth-\trimedge}{\stockheight-\trimtop}}%
        }
    \else
        \expandafter\def\csname pgf@sh@np@current page\endcsname{%
          \def\southwest{\pgfpoint{\trimedge}{\stockheight-\trimtop-\paperheight}}%
          \def\northeast{\pgfpoint{\trimedge+\paperwidth}{\stockheight-\trimtop}}%
        }
    \fi
    \expandafter\def\csname pgf@sh@nt@current page\endcsname{{1}{0}{0}{1}{0pt}{0pt}}
    \expandafter\def\csname pgf@sh@pi@current page\endcsname{pgfpageorigin}
}

\begin{document}


% Force recalculation of the current page node whenever the overlay option is used.
% If you page is centered on your stock paper it is only necessary to calculate
% the current page rectangle once.
\pgfkeys{/tikz/overlay/.add code={}{\setpagenode}}

\HUGE Verso page

\tikz[remember picture,overlay] {%
    \draw[thick,red, decorate,decoration={snake}]
        (current page.north west) rectangle (current page.south east);
}


\newpage
\HUGE Recto page

\tikz[remember picture,overlay] {%
    \draw[thick,red, decorate,decoration={snake}]
        (current page.north west) rectangle (current page.south east);
}

\end{document}