\showframe

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

 \setupbackend
   [level=0]
   
\starttext

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

\setuppapersize[TestA][TestB]

test

\stoptext