Although rules are made to be broken, try to keep some consistency in your slides.
If it doesn't fit, change the way you say it, not the font size.
Assume each slide will take about the same amount of time to present. Are they fifteen minute slides, or two minute slides? At a minimum allow 20-30 seconds per “thing” (bullet point, icon, image, etc)
Although your story will always be more important that your slides, there are awards for supporting actors (and there are actors who will upstage you).