Why can't I change size and font in my messenger.?

maybe you haven't noticed a "T" symbol there, click it and you will see all the preferences there.

if you a have trouble reading text in the internet, you might as well try this: if you have a mouse with a scroll in the middle of the left and right buttons, press ctrl while you scroll the scroller, in this way you could adjust the size of the screen. :D

what version have you got? if you have the new one when you have a conversation window open go to tools at the top and select text size

This tutorial will help you customize your font http://help.yahoo.com/tutorials/ms8/mess/im_pconv1... It applies to both versions 8 and 9.