Fonts larger than \Huge?
You can use the Memoir document class. It provides two things that are relevant to your question:
More Base Font Sizes
The standard LaTeX document classes only allow you to choose 10pt, 11pt or 12point as the "base" font size for your document. Memoir provides many more choices: 9pt, 10pt, 11pt, 12pt, 14pt, 17pt, 20pt, 25pt, 30pt, 36pt, 48pt and 60pt. Since all font size declarations are affected by the base font size, using a bigger base font size will make \Huge render in a bigger font.
The \HUGE Font Size
For when it absolutely has to be bigger than \Huge, crank it to 11 with \HUGE.
If you use Type 1 fonts (e.g., package mathptmx or mathpazo), you can simply use the \fontsize command with large point sizes:
{\fontsize{50}{60}\selectfont Foo!}
(The first parameter (50) is font size. The second parameter (60) is line spacing. An appropriate line spacing depends on the font. Something like 1.2 times font size is commonly used with CM fonts. But it does not really matter if you are typesetting just one line of text.)
A quick search on CTAN turned up anyfontsize. To quote the description:
The package allows the to user select any font size (via e.g. \fontsize{...}{...}\selectfont ), even those sizes that are not listed in the .fd file. If such a size is requested, LaTeX will search for and select the nearest listed size; anyfontsize will then scale the font to the size actually requested.
Similar functionality is available for the CM family (type1cm), for the EC family (type1ec), or for either computer modern encoding (fix-cm); the present package generalises the facility.