The font size you specify in the Preferences (18 for standard) will be used only if the font size is not specified by the web page. Nowadays most of webpages (including this Discussions) specify fonts and sizes to use in the CSS file, and the setting in the Preferences will be ignored.
As andyBall writes, you can set the minimum size of fonts in Safari-Preferences-Advanced. But the results may not satisfy you (wrong line spacings etc.).
In the same Preferences, you can specify your own CSS file, as explained in the following post by Tom Gewecke:
Re: Setting default font size doesn't stick
But here again, line spacings etc. are not adjusted correctly. To write a detailed personal CSS file by yourself is very tedious, if not impossible.
As you might know, you can enlarge the fonts (and spacings) by hitting "command +" (press-hold the command key and hit the + key, or the = key; it seems you need not to press the shift key). I will be very pleased if someone tells me how to make this "enlarged" state the default.
PowerMac G4 Mac OS X (10.4.8)