If you want to resize your text box and don't want the font size to change, you can lock the font size on the right panel in the 'Advanced Editing' screen. If you have used one of the 'Quick Videos' templates, then you are already on the 'Advanced Editing' screen.
Here are the steps:
Step 1: Select the text box.
Step 2: On the right panel, set your required font size.
Step 3: Click on the lock sign.
Step 4: If you want to enable auto-resizing, click the lock again.