As mentioned in the page TextField API apply styles to the InputProps which applies style to the input element.
Here is the code:
const styles = {
container: {
display: 'flex',
flexWrap: 'wrap',
},
textField: {
width: 300,
margin: 100,
},
//style for font size
resize:{
fontSize:50
},
}
<TextField
id="with-placeholder"
label="Add id"
placeholder="id"
InputProps={{
classes: {
input: classes.resize,
},
}}
className={classes.textField}
margin="normal"
autoFocus={true}
helperText={"Add an existing id or select "} />