Why I change font sizes
A commentator on my recent entry on how programs should make it easy to change the font size asked me to explain why I'd do this and said that they didn't even change the font size in Firefox, which boggles me quite a lot. When I boil it down enough, there are two reasons.
The first reason is the reason that I keep changing font sizes in Firefox: because (web) designers are crazily in love with not merely small font sizes, but absurdly small font sizes. I cannot count the number of websites that think that ridiculously small fonts are not only readable but desirable. More rarely there are websites that set things in fonts that are too big, at least on my machine. To a certain extent, this problem infects the designers of other browser-like text interfaces, although generally it can be fixed by adjusting the system default font size.
(Note that setting a minimum font size is not the answer for reasons I wrote about here.)
The second major reason is that my display isn't big enough to include everything I want to include at my default font size (ie, the font size that I find most readable). Since some things are less important than others, I am willing to reduce their font size in order to fit more text into the same physical space. While web designers may come to their senses, this problem is likely to be eternal.
There are two additional reasons that come up periodically. Sometimes I am vaguely watching information that is generally unimportant; because it's unimportant, I want to have it in a relatively small font size not just so that it takes up as little room as possible but also to de-emphasize it so that it stops drawing as much of my attention. Next, sometimes I am working with something that has a fixed size in characters, such as an 80x24 interface (because I am talking to something over a serial line and it knows that I have an 80x24 VT100). In this situation I often might as well crank up the text size for maximum readability and emphasis (generally the larger a window is, the more attention I pay to it).
Looking back at my first entry, most of these reasons apply primarily to browsers (broadly construed) and to terminal windows. Ignoring HTML and rich text content, editors and mail readers and the like are much more a case where I will set a text size once and then stick to it, although this is partly because my normal editors and mail readers don't give me any convenient way of detaching a content window and then shrinking it down because I'm just going to use it as reference for something else I'm doing.
(My broadly construed version of browsers is 'any program that lets the content designer monkey around with the text size and design'. If the content designer can shrink the font size, sooner or later they will and sooner or later I will disagree violently with that shrinking.)