Came up with some solution using lua, probably not the most elegant one but seems to work. Attached below in case it’s of any interest for anyone.

\setupheads[
  align=center]

\startluacode
  userdata = userdata or {}

  userdata.test = function(n)
     local breakline = n:find([[\\]])
     local modified
     if breakline then
        modified = [[{\tfa ]] .. n:sub(1, breakline + 1) .. '}' .. n:sub(breakline + 2)
     else
        modified = [[{\tfa ]] .. n .. '}'
     end
     context(modified)
  end

\stopluacode

\define[1]\MyCmd{\ctxlua{userdata.test('\luaescapestring{#1}')}}

\setuphead[subject][style=\tf,
  deeptextcommand=\MyCmd]

\starttext

\startsubject[title={A very very very long title for subject\\
we know will not fit in single line of output but\\
 A very very very long title for subject we know will not fit\\
 in single line of output}]

Some text.

\stoptext

From: josephcanedo@gmail.com
Sent: ‎Wednesday‎, ‎October‎ ‎28‎, ‎2015 ‎7‎:‎19‎ ‎PM
To: ntg-context@ntg.nl

Hello all,

Perhaps a more meaningful example, actually I explicitly break lines Inside title text.

\setuphead[subject][align=center]

\starttext

\startsubject[title={First title line\\ \tf eventual second line}] % second line same size as normal text Howto ?

Some text here

\stopsubject

\stoptext

I tried with \tf but it does not change the size of text in the title after it. (In LaTeX I used \normalsize)

Is there any solution that would allow to type:

\startsubject[title={First title line\\ eventual second line}]

and output second line in smaller text than the first one (without adding any font modifier in the title if possible) ?

Many thanks for any help
Best regards

Joseph

From: josephcanedo@gmail.com
Sent: ‎Tuesday‎, ‎October‎ ‎27‎, ‎2015 ‎1‎:‎29‎ ‎PM
To: ntg-context@ntg.nl

Dear all,

Still playing with \setfirstline, I was wondering how to apply it for headings as well ? Naive sample below does not work for the subject title, most probably because headings/titles have different processing than normal paragraphs.

\definecolumnset[TwoColumns][n=2]

\definefirstline[bbigline][alternative=line,
  style=\tfa]

\starttext

\startcolumnset [TwoColumns]

\startsubject[title={\setfirstline[bbigline] A very very very long title for subject we know will not fit in single line of output}] % first line with different style as in the text below ?

\setfirstline[bbigline] Some paragraph content here. First line should be printed bigger than rest of lines.

\stopsubject

\stopcolumnset

\stoptext

Thanks a lot,
Best regards

Joseph Canedo