I don't think this has anything to do with msgplus but I am using that as well, basically what happens is when I change my font size to 8, it keeps on reseting itself back to 10 no matter what I try. I'm on vista ultimate by the way {yeah I know, it does suck }
I tried doing it before opening a contact window then opening one and typing, no luck.
I tried changing it, rechecking it was changed, exiting live messenger, going back in, checking it still said 8, opened a contact window and BLAM, back to 10 again.

I asked our overlord google about it and he did come up with something where someone seemed to be having the same issue but I don't understand what the guy has said he did to fix it, anyone care to translate ? Look here.