Some fonts change size after sending

Some fonts in my test email are smaller than in my draft email while others (with identical formatting) are the size as intended. I realize that the correct text is assigned to "Body text" (under the "Design" tab) because it changes in my email when I adjust that parameter. How do I assign my problem text to "Body text", or any of the other styles (Heading, Feature heading, Feature text, Article heading, Article text) for that matter?