I personally would rather keep the size of a 1080p monitor at 25" or less. 23" or 24" seems to be the ideal size for 1080p. If you keep the monitor around 23-24 inches, it will be almost exactly as crisp as a 20" 900p monitor (about 8400 PPI). If you keep it around 21-22 inches, it'll still be more crisp than a 20" 900p monitor.
If you go to 25" or higher, it will be less crisp than your 900p monitor (about 7700 PPI). 27" shouldn't be ugly, but it certainly wouldn't be as sharp as the monitor you have now.
Whatever you do, try to find out what kind of panel your monitor uses (TN, VA, IPS, etc) so you don't buy something with worse color accuracy or other specs.