The "SEO keyword" area is for typing or pasting your keywords and key phrases, for reference when constructing your page title and description.
No character counting occurs on the text held in the "SEO keyword" area. Automatic character counting occurs only when you type or paste into the following areas:
Page <title>
"Description"
The total number of characters appears in light gold lettering above the 'Title' and 'Description' text areas. The display is intentionally subtle, so it won't distract as you edit.
Your HTML Page <title> tag can be any length, but Google presently shows only 65 characters of title tags on SERP pages. The first 75 characters surely bear the primary weight. Though you can add as many words as you want, always attempt to construct a title tag that works well both as a SERP item, as a bookmark, and as the window title. Most people will see it in at least one of those places.
Presently Google shows about 150 characters of your "Description" meta tag on SERP pages. The number of maximum beneficial characters is up for debate. However, it is largely accepted that 255 characters are usefully influencial.
The menus at the bottom allow you to set the font face and font size for all fields in the tool.
You cannot set the font size for the individual areas.