Set the portal rows to the max possible for your needs. Then set the sliding/printing in layout mode under the format menu.
It isn't always possible, but usually you can print your portal records by not using a portal on the print layout at all. Instead, create a print layout based on your portal's table as a list view with data from your parent table included in header, footer and grand summary layout parts. A key part of this approach is to use a script that pulls up the records shown in the current record's portal in a found set for printing on the other layout.
The Invoices starter solution uses this method for printing invoices.