You wouldn't manually stretch the portal each time. You have two options:
Set up a second layout just for printing. Make the portal a very many number of rows in height and set it to "slide up" and to "resize enclosing part".
Don't use a portal at all when printing this data. Set up a list view layout based on the portal's table. Include fields from the table of the layout you show here in the header, footer or grand summary parts on this layout. Use a button on your current layout that switches to this list view layout and finds the records shown in your portal to make them the found set of records shown in this Print layout. (Go To Related Records can be used to both change layouts and find the portal records.)
If you look at an Invoice starter solution that comes with Filemaker 11, 12 or 13, you'll find that this second method is what is used to print the contents of the "line items" or "invoice Data" portal when printing an invoice.