I don't think there is a number published or quite frankly which parameters effect the download. What you might do it try an experiment to perform n downloads of increasingly larger size and see what the results are. I used this code from the console to make 18 requests for an increasingly larger set of records (+10 at a time - 10, 20, 30 ...) from the formula function table:
Your parameters for n and size will be larger but you can just modify the script and run it. Note that this is one of the few instances where you would use a synchronous AJAX requests to avoid one AJAX request effecting the performance of other simultaneous AJAX requests.