To customize the default site-wide, uncomment and edit the following in public.config.php:

$_LW->REGISTERED_WIDGETS['events']['custom']['ical_month_range']=6; // maximum # months to return results for in ICAL API queries that don't specify a range (default: 6)

Or, on a per-request basis you can set start_date and/or end_date to specify any range.