Re: Change font
Ah, sorry, I misread your post and thought you meant at this site. Yes, you can change the font size easily at your own Tiki site by editing the CSS file of the theme(s) you are using (styles/tikinewt.css for example) or, as of Tiki3, by adding a custom.css file to the directory of the theme you are using (in styles/tikinewt/ for example - this has the advantage of being future-proof - later Tiki updates won't overwrite your edit).
The theme stylesheets all have a font size specified as a property of the body class, so this could be edited (or re-specified in the custom.css file). Some themes also specify the font size for div#main, which includes the whole visible page, and sometimes there are specific-area font sizes. But the body or div#main properterties will affect the whole page. If I recall correctly, text in forum posts generally uses the theme's default size, in the newer themes. If you are using an older theme, then it's more likely to have gone overboard with font size specifying.
-- Gary