Hi,
The way print works is that HikaShop provides the HTML to the browser, and then the browser renders the HTML but instead of outputing it on the screen, it outputs it on paper through the printer system of the operating system.
So it's not HikaShop which selects the paper size when this happens but the browser (usually it's the user when the browser displays the print preview screen).
So the first question which comes to mind is whether or not you have the issue with different browsers ?
Also, if you edit the file order / invoice via the menu Display>Views (which is used in the backend to output the HTML of the invoice), you can see this code at the end:
$navigator_check = hikashop_getNavigator();
if ($navigator_check["name"] != "Apple Safari") {
?>
<div style="page-break-after:always"></div>
<?php
}
This adds a page break after div at the end of the invoice for browsers other than Safari.
So this piece of code might be linked to the issue you're seeing. So a test you can do is to remove that piece of code and see how it goes.