The Portable HTML Standard Edition template includes a paragraph style called NewHTMLPage, which starts a new HTML page. Map the paragraph tags that you want to start new pages to NewHTMLPage, and you're set. Typically, you use NewHTMLPage for one or more heading paragraph tags.
Note: Because a new FrameMaker document always starts a new page, mapping the tag of the first paragraph in a document to NewHTMLPage does not start a new page. (Otherwise, you'd get a blank HTML file followed by the HTML file with content.)
To get your HTML files to be a reasonable size, you need to experiment. Try mapping the major section headings in your document to NewHTMLPage. If that still creates files that are too long, map the next heading level and so on, until you get HTML files that are an acceptable size.
After changing the mapping, you can immediately see the new HTML files in your project window.
Note: The red icons indicate that the files are not yet available. To create the files, generate your project.