Native mobile styling is done using javascript. Please see this link from documentation showing javascript code snippet on how to create a style class to adjust font size
https://docs.mendix.com/refguide/native-styling-refguide#2-1-class-names
 

export const customClassName = {
	container: {
		// ViewStyle properties
		paddingTop: 5
	},
	text: {
		// TextStyle properties
		fontWeight: "bold"
	}
}