can we assign width to li

Try this CSS

ul {
 white-space: nowrap;
 height: 1em;
 width: 100%;
}

li {
 list-style-type: none;
 width: 100px;
 overflow-x: auto; /* change to hidden if that's what you want */
 float: left;
 margin-right: 10px;
}

No, you can't assign width to anything that is displayed inline, which you've set your LI to be. Instead, you most often want to use display: block; float: left; which'll allow for setting width.

Try to replace your display:inline by display:inline-block