Changing the font size in Dreamweaver CS6

Select your paragraphs & headings from the HTML properties inspector (Ctrl + F3).  See screenshot from DW CC.

Use semantic HTML tags for content.  For example, <h1> for heading 1, <h2> for heading 2, etc...  Then use CSS to style your text.

h1 { font-size: 3.5rem;}

h2 {font-size: 2rem;}

Related link:

https://www.w3schools.com/css/

DW is not going to be very much fun for you to use, if you don't unerstand the basics of HTML. For better or worse, it really expects you to know how to work with code. No matter what the marketing department at Adobe has said about it over the years, it is not a WYSIWYG layout program.

Typically, for various sized text throught your page, you would use an array of heading tags, <h1> through <h5>, which have a default size and weight given by the browsers with 1 being the largest and 5 being the smallest. You would then fine-tune the <h1> through <h5> tags by using element selectors (h1 through h5) in your css to override any of the default settings you don't like.

You can still add them strictly from Design View. First, highlight the text you want changed in Design View. Next,  with the HTML button active in your Properties window (go to Window > properties if it's not open), choose Heading 1 through Heading 5 from the Format dropdown menu.