It works the way jsreport takes your html as is and puts it into the docx file source.
When you open the Word app, it interprets the HTML.
jsreport does not influence how it is interpreted.
Perhaps you can try the docx recipe and docxHtml helper, if you need to display just a simple html.
This would let you modify the page size and orientation in the word template.
https://jsreport.net/learn/docx#docxhtml