\setuppapersize
  [A5][A4]

\setuplayout
  [width=middle,
   height=middle,
   marking=on,
   location=middle]

\setupnumbering
  [alternative=doublesided]

\setupbodyfont
  [10pt]

\starttext 

This is an A5 document. The page is mapped onto A4. There 
are a lot of mapping features present in \ConTeXt. By 
setting the \type {location} to \type {center}, we center 
this A5 page on the A4 paper. 

De default layout settings of \ConTeXt\ are meant for quick
and dirty, single sided, reports. Here we prefer a centered
doublesided layout. We let \ConTeXt\ calculate the width
and height. Say \type {\showframe} if you want to check 
this. 

\page \showframe

The default bodyfont size is 12 points, which is rather
large for A5 based documents. Don't confuse \type
{\setupbodyfont} with \type {\switchtobodyfont}; the first
sets the main document fonts (sizes) while the latter is
meant for local use. Using \type {\setupbodyfont} halfway a
document will lead to strange effects when crossing pages,
because this command also affects headers and footers. 

\stoptext 
