React PropTypes: Allow different types of PropTypes for one prop
size: PropTypes.oneOfType([ PropTypes.string, PropTypes.number]),
Learn more: Typechecking With PropTypes
For documentation purpose, it's better to list the string values that are legal:
size: PropTypes.oneOfType([ PropTypes.number, PropTypes.oneOf([ 'SMALL', 'LARGE' ]),]),
This might work for you:
height: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),