\setuppapersize
  [A6]

\setuplist
  [part]
  [after={\blank[samepage,big]}]

\setuplist
  [chapter]
  [before=\blank] % is 'preference,big' by default

\starttext

\completecontent

\dorecurse {4} {
    \part{Part}
    \chapter{Chapter}
    \chapter{Chapter}
}

\stoptext
