In Photoshop on the iPad, you cannot save a copy as PSD when your document exceeds the 2 GB limit for file size. This is a known limitation. Follow the workarounds suggested below.  

(iPad) Save a copy as TIFF

Photoshop on the iPad

In Photoshop on the iPad, export your document in the TIFF file format if it exceeds the 2 GB file size limit.

For instructions, see Save and export cloud documents.

(Desktop) Save a copy as Large Document Format (PSB)

Photoshop desktop

In Photoshop on your desktop, save a copy of the cloud document on your computer in Large Document Format.  

  1. Open the document the Cloud documents tab in the app home screen on your desktop.
  2. Choose File > Save As > Save On Your Computer.   
  3. In the next dialog, choose the Format as Large Document Format.    
  4. Click Save.

For related helpful instructions, see Manage and work with Cloud documents in Photoshop.