How to change the switch component's size in React Native?

You can resize the switch by using transform property from styling,

<Switch value={true}
style={{ transform: [{ scaleX: .8 }, { scaleY: .8 }] }}
onValueChange={(value) => {}} />

also for best result determine scaling values based on screen dimension.

To expand on what was already said, this is how you can handle screen sizes:

import { moderateScale } from 'react-native-size-matters';

...

<Switch 
   style={{ transform: [{ scaleX:  moderateScale(1, 0.2) }, { scaleY:  
   moderateScale(1, 0.2) }] }} />

If the size is a UI concern only you can add a negative margin

For example on my case i would just reduce the height (a little) to keep things aligned:

<Switch style={{marginVertical: -8}} />