Having just checked all prefs that I thought might have an effect on this, the only option I found was in the "Seeing" tab in Universal in System Preferences. There, you have a Keyboard shortcut available to turn on/off Zoom.
Sorry, but that is the best answer I can give you. Perhaps others will chime in with something better.
Best Answer
I believe it is system-wide question, rather than Xcode one. What is under Xcode control is editor font size.
But what you are asking is to change system fonts that are being used for Xcode main menu, files treeView, etc.
As far as I know, zoom under accessibility is only available option in this case.
"Apple main menu" > "System Preferences..." > "Accessibility" > "Zoom"