//Get the value of style attribute based on element's Id
var originalStyle = document.getElementById('sample_id').getAttribute('style');
var regex = new RegExp(/(width:|height:).+?(;[\s]?|$)/g);
//Replace matches with null
var modStyle = originalStyle.replace(regex, "");
//Set the modified style value to element using it's Id
document.getElementById('sample_id').setAttribute('style', modStyle);