you cannot print directly from web to the printer, because of security issues in browsers.
but you can build a .pdf file with label dimension per page and then the user only need to select the right printer (only the first time) and push print over the pdf file.
if you want to do it automatically you can build a little c# console application that query the webserver every x seconds to a specified page that send the pdf to the console application that print it for you.