While you could make that portal very large and then set it to slide up/resize enclosing part, it's not likely to be your best option here.
You appear to have these tables/relationships:
If you set up your layout based on Details, you can get a list view type report that looks like this:
Invoice Data (customer name, date of invoice, invoice number, etc.)
LineItem Data (Item ID, Quantity, price, description, etc.)
Details for LIne item
Details for Line item
Line Item Data
Fields from Invoice would be placed in a header, leading grand summary part or a sub summary part (when sorted by Invoice ID).
Fields from LineItems would be placed in a sub summary part (when sorted by LineItemID)
Fields for Details would be placed in the Body of the layout.
To produce your report you would:
A) Perform a find or use Go To Related Records to pull up a found set of Details Records for the specified invoice or group of invoices.
B) Sort the records by the fields specified for the sub summary layout parts to properly group the records and to make the sub summary parts visible.
Yes of course, perfect. Thank you Phil!