Zeny follows the text size set on your iPhone/iPad. Zeny does not have a separate button in its settings screen just for changing text size.
Change the text size
Open the Settings app on your iPhone/iPad.
Go to Accessibility ＞ Display & Text Size ＞ Larger Text.
Turn on Larger Accessibility Sizes, then drag the slider to the size you prefer.
After changing it, return to Zeny. If the new size is not reflected, reopen Zeny.
For Apple's detailed instructions, see Make text easier to read on iPhone.