The font evo uses in its message zones is not synchronized with the general gnome font properties. After the evo 1.5.2 upgrade my messages font shrunk slightly. Just to make sure I enlarged the gnome font ant all the gnome ui changed except the evo mail area. I appreciate it might have looked the same on the developper box because of different dpi/font settings. On my box it looked slightly smaller and more difficult to read. On another one the effect might be even more critical.
Looking at a recent build of 1.5.* (1.5.7): if you go to Tools->Settings..., select the "Mail Preferences" option in the sidebar, then the "General" tab, you should see a "Message Fonts" category, with a checkbox to "Use the same fonts as other applications", and controls for setting a custom font. Do you see these controls? What are they set to on your machine? Does this fix the problem?
1.5.7 is a bit better. However if you use this checkbox then change the application font to something insane (say 36-40pt) you'll see some parts of the evo UI still do not follow it (and I'm not suggesting I use this setting myself, just that it makes easier to detect UI parts that do not flow correctly - with sane big fonts it's a bit more difficult to say if a slightly smaller font is a style effect of a bug)
Please identify which parts of the UI don't follow the setting, and report it as a bug in Ximian's bugzilla (and add the Ximian bugzilla ID here).