admin/environment/php setting/memory limit
From MoodleDocs
memory_limit should be at least 16M (32M is recommended for Moodle 1.7 and 40M for Moodle 1.8 or later). Large sites may need more than 128M. PHP 5.2.x requires higher memory_limit values than previous versions of PHP. 64bit operating systems require even more memory.
