I'll figure out a better way to handle this (saved searches or configurable defaults or something).
For now, you can change /Presenters/Admin/ManageReservationsPresenter.php lines 72 and 73 to:
$startDate = $this->GetDate($startDateString, $userTimezone, 0);
$endDate = $this->GetDate($endDateString, $userTimezone, 14);
The 14 can be however many days you want to show by default