\definefontsize
  [w]

\setupbodyfontenvironment
  [12pt]
  [x=2pt]

\setupbodyfontenvironment
  [default]
  [w=0.2]

\starttext
    \ruledhbox{xxx {\tfx tfx} {\tx tx} {\txx txx} {\tfw tfw}}
\stoptext
