Yes and no.
For printing purposes, you can set a portal to "slide up", "resize enlosing part". This will shrink the portal to only the number of records displayed in the portal, but if you have more records than you have portal rows, it will not "grow" to display them
This gives you two options:
1) Set up a layout just for printing where you make the portals many rows larger than you ever expect to need and specify the sliding options.
2) Don't use a portal when printing, instead use a list view layout based on the portal's table. Fields from the related parent table can be included in this layout as well--most often in the header, footer or grand summary layout parts.
1) has the obvious limitations I've already stated
2) does not work with every possible table structure. If you can arrange table occurrences in a single chain of one to many relationships, you can base your report on the furthest table occurrence on the "many" side of the relationships and it should work perfectly. If you don't have such a set of relationships, you may still have to use portals for some parts of your report, with the limitations that go with them.
I was only concerned with on-screen at this point, and didn't really think it could be done.
But the print resize aspect is interesting for down the road.
In browse mode, you have the option of enabling a scroll bar so you can still access all the portal row records no matter how many exist in the portal. You can also set anchors on the portal so that it resizes to fit the current window size--not what you've specified here, but one more option that exists for controlling the look and feel of your layouts.