I'm not sure what you're responding to. Bob H. above pointed out a clear bug in changing the Mail list font when he went to Mail/Preferences/Fonts & Colors and changed the font setting for the Mail list (second column), and it didn't work.
My issue was the font size in the first column (sidebar), which is not controlled in the Mail app, but by going to System Preferences/General and checking the box for changing the sidebar font size. This checkbox also controls the font size for Finder (Desktop) windows sidebars (left column), where hard drives, other volumes, user Documents folders, and the like are listed. This does work perfectly for me.
I have no other suggestions for you, except for perhaps trying removing the com.apple.finder.plist file located in the ~/Library/Preferences folder, which you can access by holding down the option key while selecting the Go menu at the top of the screen. This controls the Finder window settings you have previously selected, and may be corrupted. Simply move it to the Desktop for temporary safekeeping, then if it simply copied it, vice moving it out of the folder, right click on the file in the folder to move it to the Trash (you may have to authenticate). This will force the Finder to recreate a default version of the file, which you can then remodify to your liking. If that works, Trash the copy you moved to the Desktop. If not, put it back in the folder to restore your prior settings.
Good luck.