The most likely cause for this is that you are using inline formatting (edited through the Text Editing Toolbar) that conflicts with the formatting set in your Style Designer. The reason we have added the Style Designer is to allow you to have greater control over the look and feel of your site and get a more consistent result when editing the text in the page.
The quick way to resolve this is to go to Style > Designer. Set up your default settings including text size so that all of your paragraph text is a consistent size and font. You will then be able to edit text without having your link font size change when you do inline formatting.
This part is very important: You need to clean up any existing formatting issues, highlight the text and click the Remove Formatting button on the Text Editing Toolbar (second last icon next to "HTML"). When you do this, the settings in the Style Designer will apply to your text.