Is it possible to change the font and size of the header title?
Yes this is possible. You'll just have to use the stylesheet (style.css) to find the things you are looking for and edit them there. I'm not sure what you are referring too though. Do you mean the sub intro text, the intro text or the site title? Please attach a screenshot that points out what text you want to change.
Thanks for your answer.
I hope the screenshots will help to clarify what I mean with header title.
I figured out that the color can be changed on the page itself, but wasn't sure about the size and font.
Attached files: clip1.png clip2.png
You'll have to look for the following css elements inside style.css (stylesheet)
For the homepage:
.page-template-homepage #header .header-title
For other pages: