I am designing the summer school program for university and I'm interested to calculate the market size by either top-down or bottom-up approach. Please give your best rough figures!
My suggestion is go to Coursera and Edx websites several courses relating to your area is available select such courses with verifiable certificate issued by renowned universities
Easily search DAAD (Deutscher Akademischer Austauschdienst (German: German Academic Exchange Service) on google & you can visit their website & contact them.
This institute may help you through the information & statistics you've asking for.
The size of the market depends on relative quality and relative price. The lower the price for participating students, the higher is the potential market size for the given quality level.