I am experiencing this issue as well...
Popup warning when trying to send message - and unable to send message (at all) with *6 MB* attachment. Hosting server support (iPage) has set and tested .htaccess and php.ini with a 20 Mb limit, but Roundcube still doesn't send.
Is there a setting in the Version 1.0 config file that we can add to set a [higher] post/file size ?
Thanks,
John