First check your settings in Back office -> App Settings -> Files -> Maximum allowed filesize.
Second contact your host. Probably it's your host that's not allowing anonymous uploads beyond a certain size. Often the host has the php setting for it's function mail() set to a certain limit.
If you have a dedicated server and / or an IP of your own you can setup anonymous ftp
If you don't you might be able to set up multiple FTP's. You could set one up and limit it to a certain folder.