This option is used only together with the "Use fixed font sizes" option (when checked). It specifies the base font size (in points) against which the actual fixed font sizes used in the generated HTML are calculated.

The base font size used by a template is the one specified in its default "Normal" style. Its default value is 10. (Note that the actual default style used during the generation is the one specified in the topmost template, which generates a particular HTML file as a whole.)

If a control in this template is set to generate a text with the font size N, the actual font size of that text will be calculated according to the formula:

N * baseFontSize / templateBaseFontSize
where
baseFontSize - the value specified in this option;
templateBaseFontSize - the base font size specified in the template.
So, in most cases, when you set the value of this option greater than 10, the text size in all generated HTML will increase. If the option value is less than 10, the text size will decrease.