Out of the box SharePoint does not provide a way to change the width of columns in a list/library. To achieve this we need to add the below JavaScript to the page where the list is deployed

<script type="text/javascript">
$(document).ready(function() {
    $("TH.ms-vh2:contains('Title')").css("width", "300px");
});
</script>

Though this is pretty simple to implement it is a pain if you have many lists where each list requires this custom CSS. To help ease this I have developed a ribbon control. You can find the entire solution and info in the git-hub link below

SP.ModifyColumnWidth

Quick Demonstration

Once the wsp is deployed you’ll see a ribbon control on all your document libraries. On clicking the ribbon control, highlighted in blue in the below screen shot, an application page is opened that lists all the columns in the view.

RibbonControl

On this page you can enter the desired width of each column. Leave the textbox blank in case you want to keep the width of a particular column as default.

Application Page

P.S: The solution assumes jQuery is referenced in the master page.