You can try to modify the scilab script (SCI/bin/scilab)
and pass a font argument to $SCI/bin/zterm. If zterm is
based on vte then
$SCI/bin/zterm -l fontsize -e $SCI/bin/scilex ...
should do the job (for instance try with fontsize=14), at
least this work for me.
If zterm is based on zvt (this seems to be your case) it is
another option (that you could (a priori) see with a ./zterm -h
in $SCI/bin). (Note that it is recommended to use vte in
place of zvt.).
Bruno, thanks to Jpc (the gtk scilab interface 's author)