If you highlight the text again and click on the change font it will change it – you occasionally have to make the change twice for it to register – we are looking into this.
Another reason could be that you are using Internet Explorer browser, please use either Google Chrome or Firefox browsers if using a PC. If using a Mac please use either Google Chrome or Safari.
If after checking the above, the font refuses to change size then just delete the whole line, backspace back to end of last line press control and return for a single line space and re-type your text.
By doing the above it will delete any odd hidden code that could be causing a problem.
Sometimes problems such as this occur due to text being copied and pasted into an email template which can bring across different formatting which can cause issues such as this.