Button width issue

Button width issue

I just did an UL list with 4 LI elements and I did this in CSS:
ul li {
width:200px;/*problem*/
display:inline;
color:white;
background-color:red;
border-radius:5px;
font-family:verdana;
font-size:2em;
box-shadow:0 7px 0 #600;
padding:5px 20px;
margin:15px;
}
The problem is that the width property has no influency in the width of
the li element, and if I want to do it larger I have to add padding to the
right and left but that is not good because there are different lenghts in
each box since some have more letters than others.
Why is that happening?
Thanks,