Lisbon and Sintra, Porto and the Douro, or the coast to the Algarve
Most first trips to Portugal are Lisbon plus one more place. Three or four days in the capital with a day out to Sintra need no more than the 3 GB pack, valid for 15 days. Add Porto, with the train between the two cities and a day up the Douro valley, and the week becomes a 5 GB trip: tickets, the map and photos, nothing heavy.
A two-week drive in a hire car from Lisbon down the Alentejo coast to the Algarve, with the phone navigating all day, is where 10 GB means you stop rationing. The 20 GB pack is for a month in the country and, like the 5 and 10 GB ones, runs for 30 days, which leaves room if a week on the beach turns into two.