Using PowerShell, you can quickly create a test file on your computer. I use these for testing SharePoint attachment upload sizes.
You can generate files in mb, gb, etc. easily. This example generates a 5gb test file in C:\Users\MyUserName\AppData\Local\Temp called testfile.txt
$path = "$env:temp\testfile.txt"
$file = [io.file]::Create($path)
$file.SetLength(5gb)
$file.Close()
Get-Item $path
Example of the above file uploaded to SP using drag and drop:
