I believe that you can resize the Groups and Files panel by setting the PBXTableFontSize parameter to ~Library/Preferences/com.apple.Xcode.plist . To do this, you must first install PBXTableFont.
1) Run defaults write com.apple.Xcode PBXTableFontSize 20 in the terminal, replacing 20 with whatever value you want to use.
2) Run defaults write com.apple.Xcode PBXTableFont Helvetica in the terminal, replacing Helvetica font you want to use. I'm not sure OS X defaults to Sans Serif, but Helvetica is pretty generic.
welp
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