If you need to change font title size, you should change source code in qwt library:

My directory is – /usr/src/qwt-6.1/src

# nano qwt_plot.cpp 

In 6.1 version it is line number 151:

d_data->titleLabel->setFont( QFont( fontInfo().family(), 14, QFont::Bold ) );

By default font size equal – 14, so you can change it, futher:

# make && make install

and “Rebuild All” your project.