For Market Sizing, you will need to adopt a top-down or bottom-up approach, depending on the date you have at hand. In some situations, the top-down or bottom-up approaches are equivalent to approaching the problem through demand Vs supply!
In this situation, and if you do not have any data, you can assume an average GDP per country size & region. You can either classify the world countries in categories (developed, developing and third world) or you split them by continent. Then, you can ask (or take assumptions) about the average population (or % of the world's population) and you multiply it by an average GDP per person based on where they live.
I hope this helps, please do not hesitate to reach out to me should you have any question.
All the best,