font size of ascii emails

Hi,

when I read emails which are just ascii, the fonts are too large.

By pressing Ctrl-- I cannot make them smaller (Ctrl-+ makes them even 
larger though).

Html mails are displayed ok.

I haven't found any preference which changed the font size of only ascii 
text.

Can anyone give me some help?

I use: Mozilla 1.6, Debian sarge

Thanks in advance.

Florian.

PS: When I composing this post the fonts are also too large.