Hi,
You can edit the file order / invoice.php via the menu Display>Views.
There, you have these lines:
$width = (int)$this->config->get('cart_thumbnail_x', 50);
$height = (int)$this->config->get('cart_thumbnail_y', 50);
that you can replace with:
$width = 100;
$height = 100;
in order to increase the size of the images.
Note that this will change the size of the images on both the shipping invoice and the invoice available in the backend as they both use the same view file.
If you don't want that, you would have to have such code:
$width = (int)$this->config->get('cart_thumbnail_x', 50);
$height = (int)$this->config->get('cart_thumbnail_y', 50);
if($this->invoice_type!='full') {
$width = 100;
$height = 100;
}