var btn1 = document.getElementById('btn1') ;
var btn1Style = btn1.style;
btn1Style.width = 400 + 'px';
btn1Style.height = 600 + 'px';
btn1Style.marginLeft = "200px";
btn1Style.marginTop = "300px";
https://www.w3schools.com/jsref/tryit.asp?filename=tryjsref_style_marginleft
https://www.w3schools.com/jsref/prop_style_margintop.asp