Mail looks pretty different in Lion: it's got a new layout, a variety of new interface elements, and a revamped sidebar, too.
Hints reader michaelj wasn't thrilled with that last one: He found that his mailbox icons were displayed much larger in the sidebar than he'd prefer. It turns out that Mail's sidebar icons are (bizarrely) linked to the size of the icons in the Finder's sidebar. Really. Here's the fix:
Launch System Preferences and click on the General icon at the top left. In the middle of that preference pane is an option for Sidebar icon size; you can choose among Small, Medium, and Large.
Whichever icon size you choose there will show up not just in the Finder's sidebar, but in Mail's as well. (Other sidebars, like the ones in iTunes, seem to ignore this setting.) If you discover any other sidebars that the setting affects, let us know in the comments below.