If you want to change your text you have to "Go advanced." You'll see that at the bottom where you type your replies. It will give you more options than quick reply. You highlight the text and then choose size - you can see how big they go. If there is a way to make it permanent, I don't know. I can't even get the chat feature at work to keep my color or text permanent, but going advance will give you additional options, one of which is size.