While using this information:
Will have WordPress *try* to use a higher memory limit, that will not help since its your PHP configuration on your hosting server that is set to 20MB of memory - so what you need to do is check if you have access to editing your php.ini to increase the limit.
This doc here: http://www.hostucan.com/webmaster-tutorials/increase-php-memory-limit
Explains that you open the php.ini (if you have access to it) and find the memory_limit = line and set it to something a bit larger, 32MB would be good.
Unfortunately it's more then likely you do not have access to the php.ini so you would have to wait until your hosting company fixes it for you, if they are unable to give you more then you need to move your hosting to a better company.