Here are a few things you can do to help resolve your font size issue:

1. Check your font size settings under Home > Font Settings, and make sure the font size is consistent across all slides.
2. Update the look of all placeholder objects by selecting them and applying the desired text effects.
3. Ensure compatibility by removing any unnecessary formatting specifics.
4. Copy all your content into a new presentation and start from a fresh template.
5. Reset your slide zoom setting to match the size of the text boxes.