How to restore the font size and other options to default Globally

I have messed up the font size globally. I was playing with SetOptions and I think I have accidentally changed the font setting of the FronEnd setting globally as can be seen here.

I tried to uninstall Mathematical and instal it again but that did not help. how can I reset the font size to default.

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You may want to try this: support.wolfram.com/kb/3274. I haven't tried it since I obviously don't want to lose my MMA settings. If it works, credits to @halirutan for pointing me to it in the chat. – seismatica Jul 16 '14 at 7:28

Most likely you merely set to some \$FrontEnd options unintentionally. If you can figure out what these options are you can reset them by clicking the X next to the field in the Option Inspector:
@Algohi Do you recall what exactly you were doing? Of course you don't know what went wrong, but it would help to know in what manner it happened. Were you editing style sheets? Using SetOptions? Something else? – Mr.Wizard Jul 16 '14 at 9:14