Font style - If you mean the font type (e.g.: Times New Roman, Arial) then no, you can't. There's no command which lets you change the font used in your chat messages.
Font color - See vikke's great explanation.
Font size - This is not possible, same reason as for the font style. There are some special ASCII characters which will make your text bigger, but then that character will be visible too.
Disable emoticons - As vikke said, /noicon does this. It will send the message without emoticons, so you and your contact get a message like this:
Set format command - by wj8801 on 04-04-2007 at 05:09 PM RE: Set format command - by vikke on 04-04-2007 at 05:41 PM RE: Set format command - by Matti on 04-04-2007 at 07:29 PM
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