Not sure why you are having this problem. You can force the font of everything by placing the cursor in the text area of the mail, pressing CTL-A , then from the pull-down menu select Format -> Font -> Size and then select the 10pt. or 12 pt. All of the lines including the first will have that size. You must not be using plain text for this to work.
Some more information on what you are doing may help. Are you replying to mail or initiating new mail? Are you using Word for editing your mail or the default Outlook editor?