According to McKinsey Global Payments Report published in Sep 2019, the size of global settlement market is expected in 2023 to be at $2.7 trillion. Assuming the aggregate GDP for 2023 is 90 trillion, the payment market size is 3% of GDP. https://www.mckinsey.com/~/media/McKinsey/Industries/Financial%20Services/Our%20Insights/Tracking%20the%20sources%20of%20robust%20payments%20growth%20McKinsey%20Global%20Payments%20Map/2019-Global-Payments-Map-Report.ashx
Capgemini’s World Payment Reports 2018 estimates 876.4 billion non-cash transactions in 2021.
Forex market has about 6.6 trillion daily trading volume on average. Transaction fees including netting from spreads vary from less than 1 basis point to over 7% if one uses SWIFT or Western Union, a typical international transfer from overseas citizens to repatriate their money to their home countries in many developing countries. Assuming the average fees earned collected by all the agents are say 11.74 basis point, the global aggregate forex fees would mount to $2.7 trillion.