You can change the text size in most slides. Settings related to text size can be found in the upper part of the slide menu. Right next to the text fields.
Change the text size by clicking on the menu for the text field you want to change.
Settings for text size differ with slide type. Most commonly, you can change between Huge, Huge Number, X-Large, X-Large Alternate, Large, Large Alternate, Medium, Medium Alternate, Small, Small Alternate, and Tiny text sizes.
Depending on the text size, you can fit a different number of characters into the slide. For specific settings for text size, check the features for the slide you want to use.