It is impossible to obtain the outerwidth of a div on the server side because the server lacks knowledge of the width. The actual size of an element depends on how the client interprets the HTML and CSS.
Consider using style="width: 80%"
on the highest-level element. The actual size will vary depending on the size of the browser window.
Without knowing the specific context of your query, if you truly require information about the size of elements on the server side in your own application, you could have the client convey this data to the server.
If your question pertains solely to retrieving the size of elements using Java, you can open a browser with Selenium and ascertain the dimensions using WebElement.getSize().