you are thinking of the property all wrong.
Box shadows are like the BASIC dropshadow effect in PS. The property generates a box, to which a BLUR can be ADDED ( this makes the generated image even larger than the dimension of the original element). You can also control the spread ( with that fourth value you were mentioning ), but that value will take effect all around the shadow image, and not on a specific side.
in other words, NO MATTER WHAT YOU DO , a box shadow image will retain the same aspect ratio as the box it was generated from, ( this is , BTW, the mathematical definition of the word ‘drop shadow’ )
That being said, nothing precluded you from making drop shadows of generated elements ( such as :before/:after) which are the size you want,and using absolute positioning , to place them behind your desired object.
<html>
<head>
<title></title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="viewport" content="width=device-width">
<style type="text/css">
.x, .w>div{ width: 350px;position: relative; background: pink; }
.x:after{position: absolute; top:3px; bottom:3px; left:0; right:0; content: ''; box-shadow: 0 0 6px #000; z-index: -1 }
h3, p{ margin:0; padding:10px;}
.w{overflow: hidden; margin:10px 0 }
.w>div{margin:0 10px; box-shadow: 0 0 6px #000; }
</style>
</head>
<body>
<div class="x">
<h3>This elements appears to have only <em>vertical</em> box shadow</h3>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
</div>
<div class="w"><div>
<h3>you coul also use wrappers to <em>crop</em> drop shadows</h3>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>
</div></div>
</body>
</html>
Speaking of which, you may consider the Z index (making it bigger… you will need to add position relative if it doesnt have ap/rp already)of your second box, I think the drop shad of the first is casting over your second element.
hope that helps