This is probably doable, but without an example site, it’s hard to provide specific CSS that will make the updates you want.
Generally, my approach to this issue would be to review the sites to see if there are common elements or classes being used that you can override with global CSS.
To review this, in Google Chrome, open the page with the text you want, right click on the text that you want to update, and click “Inspect.” Take a look at the code around the text to see if the text elements are using the same elements or classes, then create some custom CSS utilizing the fonts and sizes you want.
