the default [html] templates (in wppizza -> templates -> print) are set to 100% width – but feel free to edit this as you wish in the appropriate css declaration boxes there
if you have not changed the css at all then you will have to ask your printer manufacturer as to why it does not print html as instructed. however, you should make sure your html and your css is actually valid html/css (i.e all tags are closed properly etc)