add a id :

        var targetImg=document.getElementById('targetImg');
        targetImg.style.width= '20px'; 

  function createOneSnow(){
    var leftX = Math.random()*window.innerWidth;
    var topY = Math.random()*window.innerHeight;
    var snowDiv = document.createElement("div");
    snowDiv.style.position = "fixed";
    snowDiv.style.left = leftX + "px";
    snowDiv.style.top = topY + "px";
    var targetImg= document.createElement("img");
    targetImg.src = 'image/snow.png'; 
    targetImg.style.width = '20px'; 
    snowDiv.appendChild(targetImg);
    document.body.appendChild(snowDiv);
}

you can try this
if you modify it for later convenience, I suggest you write this

/**
 imgWidth --Number
*/
 function createOneSnow(imgWidth){
    var leftX = Math.random()*window.innerWidth;
    var topY = Math.random()*window.innerHeight;
    var snowDiv = document.createElement("div");
    snowDiv.style.position = "fixed";
    snowDiv.style.left = leftX + "px";
    snowDiv.style.top = topY + "px";
    var targetImg= document.createElement("img");
    targetImg.src = 'image/snow.png'; 
    targetImg.style.width = imgWidth + 'px'; 
    snowDiv.appendChild(targetImg);
    document.body.appendChild(snowDiv);
}

  
 does not understand what you are trying to say. If you just want to set the width of each picture separately, such as img1 width=20px img2 width=20px, then you should use   xsf_1991  . 
 if you want to set a different width for each picture, that is, the width of each picture is not manually set, you can write targetImg.style.width= Math.random () * 100 + 'px'; 
 on the basis of xsf_1991.