\showframe

\setuplayout
  [location=middle,
   width=middle,
   height=middle,
   cropoffset=auto,
   trimoffset=1bp,
   bleedoffset=2bp,
   artoffset=2bp]

\starttext

\definepapersize[TestA][width=100bp,height=200bp]
\definepapersize[TestB][width=150bp,height=250bp]

\setuppapersize[TestA][TestB]

test

\stoptext
