For some reason, the text size is coming up as MS Sans Serif Small when I write any mail. I've changed the settings in the program options, but it's not making any difference.
Which option did you change? There are two different ones. Most likely it's the "Default Body Format" under "Sending Mail". Is the checkbox beside "Use the following font for the message body" checked?
Also check your display fonts:
Poco 2 : Look in Tools/Options/Appearance/Fonts Poco 3 : Look in Tools/Customize/Display Fonts