There are two ways:
The first way is through simple HTML coding...
<font size="n">your text here</font>
Replace n with any number from 1 to 7 with 1 being the smallest and 7 being the biggest. However, no matter what number you pick... most Windows users can change it using the "Text Size" feature in their Internet Explorer.
The other way is to use CSS coding...
body { font-size: n pt; }
Replace n with any number you want. Yes, it can be ANY number. You can make it 100 if you want. The difference with CSS and simple HTML is that HTML has 7 pre-defined sizes. CSS has specific sizes which is exactly as the number you used. HTML can be resized, while CSS can't be resized unless users are using Mozilla-type browsers. One last thing... you can choose between "pt" or "px". "pt" is point and "px" is pixels. If you say "10 pt", that is like using size 10 in MS Word. If you say "10 px", then that is 10 pixels and smaller than 10 point. Experiment more between pixels and points. I usually use pixels.
Oh, I forgot to add... if you use CSS, it needs to be in the <HEAD></HEAD> section.