Limitation
The reason for this limitation is based on the interface of the original Tasks Report Macro. It allows to provide spaces and pages and hence expects a limited number of pages to cover. The list of provided pages is exchanged via whenever a set of tasks is rendered. The length of an HTTP request is limited, therefore we need to limit the number of pages to guarantee to not exceed the usually accepted length of an HTTP request.
In case you can guarantee the correctness of the request even with a larger number in your environment, the space property allows to push the boundaries. Please make sure to take values of other macro parameters, which are also send via the HTTP request into consideration.
The size of the set is typically only a convenience problem. The implementation of the original Tasks Report Macro does not only fetch tasks from the specified pages, but also from the children. So the query simply needs to determine the root page to cover all relevant tasks for the report.