I can't seem to find a tab to select the font and size.
(1) You can't find such a "tab" because one does not exist.
There are two ways you can address font changes in styles, size and colours.
If you have purschased the CSS Upgrade you can change all aspects of font / font size / font colour on your style sheet. The upgrade costs $15 per year per blog and is not recommended for those who have no experience with css editing.
(2) If you have not purchased the css customization upgrade you cannot change the default font your theme uses. We are all blogging on a shared multi-user blogging platform at wordpress.com so if you could change your font size default on your theme each and every other blog with the same theme would be likewise changed. That means that if you want to do this, you must do this all the time in each and every post you want to make the changes in.