Clowdz will only store them once on regular storage. If you wish to store on cold storage, please only select them under cold storage tab in the Clowdz client.
No, you can backup any file no matter how large.
Cold storage is incorporated within Clowdz; both will have the same backup settings and customization.
Cold storage is just as secure as Clowdz regular storage; your files undergo 3 levels of encryption; 256-AES machine level encryption, transferred via secure sockets layer, and stored on encrypted 256-AES Server Side encryption. The bottom line is that your files are also processed with zero knowledge.
From the Clowdz client installed on your machine, go to data selection, then select the cold storage tab and add files and folders. The Smart Selection and My Computer selections will be backed up to the regular storage.