How do I format (font, font size) plot title ?

Correct, the title will be a separate region, containing only text, for which you can set the format to whatever you like. Once done formatting you position it over the plot region at your desired position. Best may be to have the plot region overlap the text region completely. You should be able to set which region is at the foreground, the other then moves to the background.

If you add or delete empty lines above the plot, the plot and the text regions will move together.

Success!
Luc