Lynch & Horton (2001) entertain a number of considerations that are important to intermediate and advanced designers. It is not necessary to discuss this topic to the level of detail handled within their text. However, their suggestion to use sizes greater than 6-point is a good one. They also indicate there are many ways for defining text sizes. Some of the techniques render fixed sizes and some flexible.
The differences between fixed and flexible are important. The fixed methods prevent users from adjusting the sizes, which can interfere with visually impaired users. The flexible techniques offer consistency. The pages that use flexible methods will look the same from one computer to the next -- no peculiar layout surprises to the viewer.