I'm going to answer my own question, as I just discovered what's happening and I hope it helps others:
I see the Text Size option on my iPhone XS when I hold the phone horizontally. The option gets cut off when held vertically, presumably because there's not enough real estate on the screen. I'm assuming this isn't an issue at all on larger screens and devices. In any case, I'm glad I found the Text Size option and hope this explanation helps others who may have experienced the same issue.