Esther S. answered 11/27/20
Art Director and Graphic Designer in the marketing industry
As far as I know, there is no direct way to save one artboard directly to another .ai file, but you can "Save As" a .pdf and then re-"Save As" an .ai file. Another option is through copy and paste.
Option #1:
Select the current artboard you want to copy.
(Windows: Artboards: select desired Artboard)
Save As: Select Range: (Select the Arboard #)
Open new .pdf and Save As .ai file
Option #2
Make a new file will desired measurements
Go to the old file and make sure the rulers are set to default
Select all elements from the desired artboard and "Copy"
In the new document, make sure the rulers are matching the old document
Right-click your mouse: "Paste in Place" into a new document
Save the new file