\mainlanguage[es]
\definehead[footnotetitle][title]
\startsetups[chapter:after]
\ifcase\rawcountervalue[footnote]\relax
     \else
         \startfootnotetitle[title={Notas}]
             \placefootnotes
         \stopfootnotetitle
     \fi
\stopsetups

\setupnotes[location=none]
\setupnotation[way=bychapter]
\setupfootnotedefinition[style={\switchtobodyfont[12pt]}]

\setuphead[chapter][aftersection=\setups{chapter:after}]

\starttext

\startchapter[title={Chapter One}]

Here comes \footnote{a footnote}. Aquí \footnote{otra nota a pie de página}.

\stopchapter

\stoptext

However, I'm concerned by \switchtobodyfont. How should I make footnote size the same as the default font size, even when an arbitrary font size is used.

E.g.:

\setupbodyfont[24pt] % It could be another number instead of 24

\setupfootnotedefinition[style={\switchtobodyfont[???]}]

Thank you in advance.

Jairo :)