Google have made it a little easier to handle relative font sizes within Google Docs and Slides.
Now, if you highlight some text (or a number of text boxes) in Docs or Slides, you can manipulate the font size of the highlighted text in increments of 1pt.
To do so, select Format > Font size > Increase font size or decrease font size from the menu bar.
Note that If the selected text includes multiple font sizes, each size will be increased or decreased accordingly, maintaining their relative differences in size.
There are keyboard shortcuts too (on Mac, replace Ctrl with Cmd):
Increase font size: Ctrl + Shift + >
Decrease font size: Ctrl + Shift + <