When this happens to me, I select the text and change it to an enormous size --6 =(24 pt) and the re-select the 6 type and change it back to 3 (12pt) . That seems to work.
If, on the other hand, I select the text and immediately and change it to 3, it stays tiny. I have no theory for why this works, and I'am simply content that it does.
Hope this goofy work around helps,