Not exactly the same thing as changing what it says. Changing the size of the heading text would require CSS editing experience and the paid CSS upgrade. Search the FAQ’s or the support pages for “CSS” for more information on the upgrade and what you can do with it.
Probably by subtitle skpr means the tagline. It is in dashboard> general> tagline (just below the blog title)! All themes may not show the tagline though. You may need a theme that shows the tagline (all show the blog title of course).