A hack to /moodle/admin/settings/server.php

line 177 begins // performance settingpage
options in the pick list are hard coded beginning
line 185
One could add lines in the array.

'1536M' => '1536M',
'2048M' => '2048M'

make sure the line above the entry for 1536M ends with a ','

Values added will show in the pick list.  Probably don't have to warn about hacks and updates, but just did. ;)

Another way ... add a line to config.php file:

$CFG->extramemorylimit = 'YOURVALUEM';

And for future reference, just about anything in mdl_conf table could be added to config.php.  Might be better than the hack - easy to change.

'spirit of sharing', Ken