The following script does not seem to scale the font in the enhanced text:
set term x11 enhanced
set title "Big Title\n{/*0.5subtitle}"
plot sin(x)
Note that it works with postscript:
set term post enhanced
set output "foo.ps"
replot
I'm using gnuplot 4.6.0 on Ubuntu Linux.
That probably means your x11 window is defaulting to a non-scaleable font.
I attach a screenshot of the output on a correctly configured linux system.
See "help set term x11 x11_fonts".
screenshot of output from above commands