Select Default to remove previously applied fonts; Default applies the default font for the selected text (either the browser default font or the font assigned to that tag in the CSS style sheet).
HTML font sizes are relative, not specific, point sizes. Users set the point size of the default font for their browsers; this is the font size that they will see when you select Default or 3 in the Property inspector or Text > Size submenu. Sizes 1 and 2 will appear smaller than the default font size; sizes 4 through 7 will appear larger. Also, fonts generally look larger in Windows than on the Macintosh, though Macintosh Internet Explorer 5 uses the same default font size as Windows.
These numbers indicate a relative difference from the basefont size. The default basefont value is 3. Thus, a +4 value results in a font size of 3 + 4, or 7. The maximum sum for your font size values is 7. If you try to set them higher, they appear as 7. Dreamweaver does not display the basefont tag (which goes in the head section), although the font size should appear properly in a browser. To test this, compare text set at 3 and text set at +3.