What am I doing wrong to make the font change?

I know how to change the font. My problem is that while Im chatting in my IM the font gets bigger and bigger. Or it gets small and almost clear. What can I do to fix it? And what am I doing to make it do that?What am I doing wrong to make the font change?
Sometimes it depends on which font the one you're chatting with uses.



CHANGE FONTS IN YAHOO INSTANT MESSENGER:

With Messenger open, click on Messenger.

From the drop down menu, click on Preferences.

Click on the General Tab and select Appearance.

When this page opens you have the option to change fonts for Yahoo Instant Messenger.

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