How to style FormControlLabel font size

Use material box fontSize instead of giving external style.

 <FormControlLabel
        control={<Checkbox name="checkbox" />}
        label={
             <Box component="div" fontSize={15}>
                Small
              </Box>
        }
  />

FormControlLabel exposes typography as prop. tested and works in Mui V5. https://mui.com/api/form-control-label/#props

<FormControlLabel
  componentsProps={{ typography: { variant: 'h3' } }}
/>

You could define the label as a Typography component and apply the style there:

<FormControlLabel 
    control={<Checkbox value="Hello" color="primary" />}
    label={<Typography style={styles.formControlLabel}>World</Typography>}
/>

UPDATE:

As per Saber's comment, newer versions should instead use:

<FormControlLabel 
    control={<Checkbox value="Hello" color="primary" />}
    label={<Typography className={styles.formControlLabel}>World</Typography>}
/>

Use overrides section in theme.ts

export default createMuiTheme({
  overrides: {
    MuiFormControlLabel: {
      label: {
        fontSize: 14,
      },
    },
});