Common stuff

Component-Component.js

import sizePropType from '../propTypes/size';
import sizeClass from '../utils/sizeClass';

const Component = ({ size }) => {
    let classNames = '';
    if (size) classNames += ' ' + sizeClass(size);
    return <div className={classNames} />;
};

Component.propTypes = {
    size: sizePropType.isRequired
};

Component.defaultProps = {
    size: '4'
};

export default Component;

propTypes-size.js

import { PropTypes } from 'react';
export default PropTypes.oneOf(['1', '2', '3', '4', '5']);

utils-sizes.js

export default function (size) => 'S' + size;

*** completely un-tested