How can I change font size and font style in my emails?

You might need to elaborate a bit more. Who is your email provider? How do you read and write your emails? Is the question about the presentation just for yourself, or you intend to change the formatting of the emails for everyone reading them? Is that only for reading, or also for writing, i.e. actually formatting the mails you type with some font, to then be sent out with your email provider?