Mail received a revamped sidebar in Lion, and some of us still aren’t thrilled about it as the mailbox icons display larger than we’d like. You can adjust the size of the icons – but the workaround isn’t intuitive:
Launch System Preferences and choose General at the top left. In the middle of the preference pane is an option named ‘Sidebar Icon Size’; you can choose Small, Medium, or Large.
Whichever icon size you choose there will show up not only in the Finder’s sidebar but also in Mail’s. (Other application sidebars, like the one in iTunes, ignore this setting.)