You need to increase the PHP directives below from the php.ini (/etc/php.ini) file.
upload_max_filesize
post_max_size
Increase those two according to your preference.
Then restart Apache for those changes to take affect.
There's also a setting in Horde regarding attachment size. Make sure to set it to zero or no limit.
It's "attach_size_limit" option in /etc/psa-horde/imp/conf.php.
$conf['compose']['attach_size_limit'] = '0';