Website Text
When it comes down to the wire, text is the mainstay of a website and the focus should be on readability.
The absolute minimum size for text content is 14px. In general , use 15px for Verdana and 16px for Helvetica.
Using a font size lower than 14px will cause overspill in some browsers. If you must use a font smaller than 14px, do it in the footer where the messed up layout will be less conspicuous.
Line height is important too and the value used will depend on the font family and size. Adding a little extra line height lets the text "breathe" and makes it easier to read.
Readability
The readability of text is effected by hierarchy, contrast, line height, letter spacing, line length and alignment.
Hierarchy is taken care of by using the various sizes of HTML headings.
See below for info about hex color numbers and how to get them.
For maximum readability, use a line height of 22px and do not adjust letter spacing.
Break up large chunks of text with a single or double line break.
Avoid aligning text "center" and never justify it. Justified text causes problems for those with reading difficulties and makes it difficult to follow. The ragged right hand edge of the text block actually makes it easier for the eye to find the next line.
Line Length
With responsive pages it is important to consider the length of the lines of text. If the lines are too long, it is difficult for the eye to jump from the end of one to the begining of the next.
In general, the maximum width for a text section should be no more than 1000px. If the sections need to be wider, consider using a text wrap widgets like the one used below or, in no image is required, use auto columns.