Copy-pasting text from MS Word to Mac Mail does not preserve formatting
The easiest solution is that you shouldn't use Word to compose your email.  Use Apple Mail, or Outlook:Mac, or whatever your email program is.

Why is this?


Email is formatted using HTML.  Word produces very bad HTML that is highly likely to not look the way that you want it to, especially when you send it to someone else.  
There are two ways that font sizes are displayed: in points and in pixels.  The world has mostly settled on points for specifying font sizes.  When you see that there's a huge difference in font size when someone responds to your mail, it's because their mail program thinks that your font size was in points when it was actually in pixels (or the other way around).  


If you're using Word because you really want specific formatting that is possible in Word but that you can't do when trying to format it that way in your email application, this probably means that the formatting that you want to use isn't appropriate for email.  You should either use formatting that is appropriate for email (which doesn't extend far beyond choosing from a handful of fonts and font sizes, as well as some basic formatting like bulleted lists and bold/italic), or you should send your highly-formatted item as an attachment (either a Word document, if you're sure that your recipient has Word, or a PDF).