React Native: is it possible to adjust fontSize to make sure font fits in one line?

Yes. You can use pixelratio and dimensions from react native Use it as below

import { Dimensions, Platform, PixelRatio } from 'react-native';

const {
  width: SCREEN_WIDTH,
  height: SCREEN_HEIGHT,
} = Dimensions.get('window');

//use your testing phone width to replace 320
const scale = SCREEN_WIDTH / 320;

export function normalize(size) {
  const newSize = size * scale 
  if (Platform.OS === 'ios') {
    return Math.round(PixelRatio.roundToNearestPixel(newSize))
  } else {
    return Math.round(PixelRatio.roundToNearestPixel(newSize)) - 2
  }
}

For the box style, use the above method as follows.

const styles = {
  textStyle: {
    fontSize: normalize(12),
  },
};

Above method is more accurate when you want to dynamically change the font size.

But if you only want to fit the text to a single line you can also use adjustsFontSizeToFit as follows

<View>
  <Text
    numberOfLines={1}
    adjustsFontSizeToFit
    style={{textAlign:'center',fontSize:50}}
  >
    YOUR_TEXT_HERE
  </Text>
</View>