Currently set to 100.

This is because originally the screens had hidden elements to hack around PHP forms and checkboxes (to see if they were ticked, not ticked).

The hidden elements had the same name as the actual checkbox elements. The work around for max_input_vars (hard setting for php forms to have 1000 elements only) doesn't kick in unless specified and also doesn't see 2 elements of the same name.
Workaround is in lib/setuplib.php line 943
Form declared in grade/report/singleview/classes/local/screen/tablelike.php line 194

In MDL-18229 Damyon and I removed the hidden elements around the checkboxes and now we feel we can increment the hard-limit of 100 for items per page to lower the amount of pagination required in M sized or greater courses.