Test your site over a modem. If you get frustrated waiting for graphics to download, chances are your users will, too. You can also import Netscape's color palette into Photoshop and use it. If you use the proprietary tag, use the spec-compliant as well. And so on for style sheets and background graphics and tables and text alignment attributes.
Users customize their desktops and set preferences in applications all the time.
So, if users want to incr ease the type size for easier readability, let 'em. Your users stand to gain everything.