Changing the text size is not working

The only sizes supported by the document class option in the standard classes are 10pt 11pt and 12pt. To get text at other sizes you can use commands such as \large or \tiny within the document. Normally though you should not be using font size commands within a document. Heading commands such as \section and other structural markup should be defined by the class to use an appropriate size. One of the aims of LaTeX is that you should as far as possible be using structural markup such as \section and \subsection and not using explicit font and size changes, which make it a lot harder to reformat your document for different purposes.

You may already have a document class with the ext prefix on it (such as extarticle instead of article as the document class. Give it a try, it works for the font sizes 8pt, 9pt, 10pt, 11pt, 12pt, 14pt, 17pt, and 20pt.

These document classes are available from the extsizes package, included in the Ubuntu package texlive-latex-recommended.