I'm trying to modify my text so that lowercase letters are uppercase letters just smaller. Is there a way to do this in Illustrator, without manually changing the font size?
The term you're looking for is "small caps". Type designers sometimes make versions of their fonts that are small caps, or have them as alternates within the regular font.
If the font does not have small caps, many programs can simulate small caps automatically just by setting uppercase a bit smaller. So you have 2 options for setting small caps in illustrator: