Bruges, Ghent, Antwerp or the European quarter: which size
Most visitors see Belgium by train from a base in Brussels, with day trips there and back. A long weekend in Brussels with a day in Bruges or Ghent fits in the 3 GB pack, valid 15 days. A week of outings to Bruges, Ghent, Antwerp and Leuven, with timetables, maps and photos, is what the 5 GB for 30 days is for.
Work trips change the numbers. If you are in the European quarter for meetings and the laptop will run on your phone's hotspot, take 10 GB: a laptop syncing files and sitting in video calls uses far more than a phone. A weekend in the Ardennes, or a race weekend at Spa-Francorchamps, goes back to 3 GB.