How to set the page layout dimensions in ConTeXt?

The typesetting area in ConTeXt comprises twenty five areas:

The cutspace is missing in the figure. You only need the text area and the footer (for the page number). The setup is:

\setuppagenumbering
  [alternative=doublesided, location=footer]

\setuplayout
  [header=0pt,
   margin=0pt,
   footerdistance=1cm,
   topspace=1cm,
   backspace=1cm,
   cutspace=8cm,
   width=fit,
   % margin bottom of text to paper
   height=\dimexpr
    \paperheight-\topspace+\footerheight+\footerdistance-5cm\relax]

\starttext
  \showlayout
  \null
\stoptext

\setuppagenumbering[alternative=doublesided] switches to the mirrored layout on even pages. When the top area and the header is set to zero topspace states the distance from the top of the paper to the text area and backspace the distance from the left to the text. cutspace is the distance from the right of the text to the paper. To make it work width=fit needs to be set.

The distance from the bottom of the text to the paper cannot be set directly but can be computed. The 5cm in the height= line states the distance from the bottom of the text to the bottom of the paper.

See also ConTeXt wiki - Layout and ConTeXt wiki - \setuplayout.