upload_max_filesize

Unless you have your own server, your host will have to up it.
You can try this thread for suggestions:
http://wordpress.org/support/topic/56654?replies=12

This site may help:
http://www.radinks.com/upload/config.php

And this one:
http://www.mcs.vuw.ac.nz/technical/software/PHP/configuration.html

I’ll give you an .htaccess directive that should work. Put this in the .htaccess of your root:
<Location ‘/seperate_directory’>
php_value upload_max_filesize 128M
php_value post_max_size 128M
LimitRequestBody 134217728
</Location>
Replace separate_directory with your WP directory.
Warning, your host may not like this! Talk with them first.