I was struggling with this for a week. My 12px fonts were stuck at a DPI of 300 and I had no way to adjust them.
I also had an older copy of Photoshop Elements 6, so my options I thought were limited. I even re-installed Photoshop but that did not succeed, how did I finally fix this problem?
I went to my Library/Preferences and deleted all things Adobe Photoshop Elements. I restarted Photoshop and the fonts went back to normal size. I’m not sure what file in the preferences were corrupt, but that should fix your problem.