If you find the icons and text in the sidebar of iTunes, Mail or the Finder to be too small or too large, you can change their sizes. But the setting isn’t easy to find; it’s not in the preferences of any of those three apps; it’s in the General pane of System Preferences.
Choose a size from the Sidebar icon size menu: you can choose Small, Medium or Large. Here’s how the three sizes look in the iTunes sidebar:
Note that the above screenshots are a bit larger than what you’ll see, depending on your display, but the scale between them is accurate. I scaled them at 50% from screenshots taken on my retina iMac.
This setting applies to other Apple apps, such as iPhoto and a few others. Try the setting in each of the apps you use; you’ll find the right size for you. However, it’s a global setting, and if you want to use Small in iTunes and Large in the Finder, you’re out of luck.