Question about Outlook Express

Hi, I have been over a number of other forums but can't seem to find an adequate answer to my problem. I use Outlook for MAC 2011 based on OS X v 10.8.2, when sending emails to Windows based Outlook, they are reading with different font types and sizes. My standard is Arial pt 12, but emails are being sent with a mixture of font types and sizes. Also, when I reply to an email that has been received the font size defaults to font 16 and even through I correct it, it still gets sent with 16. I have reviewed a number of different solutions, e.g. compose in Text Editor to downloading Thunderbird and using that! Anybody got a solution, I can't use Outlook now, am having to use Webmail to send emails...