mini-tutorial: My text appears too small or too large - Photoshop
also, if you find any inaccuracies I'll gladly accept corrections. In no way do I claim to be a know-it-all.
Why does my text suddenly appear very small/very large and how can I fix it?
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Most of the cases the problem resides in the image's resolution.
Computer graphics (meant to be exposed only on the Internet) do not need to have a high resolution, as monitors will display at 72 pixels/inch anyway, so you've probably worked with this resolution without even worrying about it, until you've found a pic that is, say, at 300 pixels/inch, or 1 px/inch.
Suddenly the font you knew so well displays horribly stretched, or you have to get it to gigantic sizes to make it show up; you may think it doesn't appear at all if the image's resolution is too big.
Just change the resolution: Image -> Image Size
a) un-check Resample Image; this way it won't modify the displayed image size; you'll notice, maybe, that it changes the "Document size" - if you were to print it; I never print my icons, so...
b) put 72 pixels/inch for Resolution
About text measuring in points vs. pixels: I've only worked with points; if for some reason you want to switch to another measurement unit:
Edit -> Preferences -> Units and Rulers
Type: a drop-down menu where you can choose between points, pixels and millimeters.
If you have any questions regarding this problem, reply to this entry and I'll try my best.