Often the font size will change at will. Sometimes it's because there is a lot of text in that area, sometimes it is when you add new text, and sometimes, just because it can. I have also noticed it happens quite frequently in the preview emails. And when I try to correct the size by highlighting the area and hitting the drop down bar, it won't correct. This is also sometimes true with the italics and bold features. It would be lovely to have this bug sorted out. Thanks so much.
I'm sorry to hear this is happening! I can see you're still working in our older Legacy Newsletters. Are these campaigns that are being copied and reused? Are you copying and pasting text? We have seen in previous Legacy Newsletters where pasting text or reusing templates too often can cause problematic formatting. If you are copying and pasting text, I would suggest following these steps to remove problematic formatting from your copied text. Other solutions we suggest are creating new blocks, using our clear styles tool, or even moving over to our Third Generation Editor. The newsletters in this new editor were designed to help avoid formatting issues like these.