A lot of this is personal preference. I personally had a 24" 1920x1200 monitor and I found reading text, while browsing, was difficult. I always would set text size to about 120%. I now have a 120hz, 27" monitor @ 1080p, and now I usually set the text size when browsing the web, to 105%-110%, but for the most part, the size of icons and text is a lot closer to good at 27" for a 1080p monitor.
For gaming, everything is still very crisp. Keep in mind, that what ever size you pick, after a week, it'll feel completely natural.