Other than doing what Guru mentioned above, I don't know of a way to automatically reduce the font size in Yahoo Classic. If the two regular senders are friends, perhaps asking them not to use 20 pt font size would be the best way to go, as the sender is determining font size when they compose the message.
I checked to see if you could set the font size with regular Yahoo mail and it doesn't even look like that is possible unless the sender used plain text to compose the message, "You can change the appearance of text in email messages you send and receive that use plain text. If a message you receive contains HTML code or rich text, you won’t be able to choose the font or the size of the text."
Found that here: http://help.yahoo.com/tutorials/cg/mail/cg_receive1.html