material ui: how to change fontSize in Lists?

For mui v5, use primaryTypographyProps prop to pass desired styles

<ListItemText 
    primaryTypographyProps={{fontSize: '18px'}} 
    primary="List Text"
/>

As the docs of ListItemText says you can override the primary text styles using classes props and primary key.

Create a style

const styles = theme => ({
  listItemText:{
    fontSize:'0.7em',//Insert your required size
  }
});

Apply the style to ListItemText

<ListItemText 
            classes={{primary:classes.listItemText}}
          primary="Inbox" />

If you want to override the secondary font size apply styles to secondary key.

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