Please verify the size of this folder on my C: drive

[Sal_gal]

I have Win10. I have a folder, Documents, where I store all my documents. I have an external drive that has documents files saved and I cannot access the files through my computer ports.
Thus, I want to buy a flash drive that I can transfer documents to. I did a Properties size and it shows 980GB. That seems absurd. Here is what the readout states:

Size: 980 GB (1,052, 720,369,776 bytes)
Size on disk: 983GB
Contains: 1,458, 689 files

That seems like a lot of files and I wonder if some of them are duplicates. I'm trying to clean up the Documents & C: drive so I can transfer files to a flash drive vs. an external drive. What size flash drive should I buy? Thanks.