Hi @swarnodigital,
In order to print on thermal printers, you have to use the code snippet you will find in this comment: https://github.com/wpovernight/woocommerce-pdf-invoices-packing-slips/issues/157#issuecomment-1265208166
That code snippet will make dynamic the PDF height, in order to print in continuous paper, which I guess is what you meant when you said “thermal printer”, right?
Please note that we do not offer small page sizes by default, because our default Simple template is not prepared for small layouts, but you can always set your own customizations by your own. See: Use custom page size/orientation & Using custom styles. Also, here you have an example I sent to other customer (2nd code snippet): https://projectdmc.org/support/topic/can-i-also-print-the-invoice-bill-with-a6-thermal-printer/#post-17331681