I'm fairly sure 6400 dpi is overkill, for most purposes and will produce images requiring a great deal more memory to save. The resolution of the images I have (and I only have three) are all 72dpi and with the following dimensions:
2660x1708
2603x2071
1920x1280
One is a scanned newspaper page that was scanned and it looks quite good in the storyline, even scaled down to Fit (Spatial Conform). [1920x1080 project.]
Scanners use a resolution that is dots per inch to be printed on paper in the real world. Televisions and monitors use a screen resolution of something like 1920 pixels across by 1080 pixels down and these dimensions are spread across the size of the television/monitor. The same number of pixels has a greater PPI on a small monitor and a much lower PPI on large screens (that's why 4k's are popular.) [The larger the target monitor, the more dpi you're likely to need.]
Most monitors fall within about 90-250 ppi (pixels per inch) and your scans probably should at least match that resolution (300dpi should be more than enough as long as the dimensions of the scanned image fill the dimensions of your video projects.) Mac "retina class" monitors are 218ppi (or at least, that's what they've been... no telling what the next gen will bring.) iPhones have a much higher ppi resolution (401?).
Screen size and pixel "density" will have an effect on your final results. Scaling up if you need or want to will also influence the DPI you will need for clarity.
You might be able to find some online resources to help with calculating the DPI needed for PPIs of various monitor sizes.
I'm sorry I cannot give you a more precise answer.
Here's a current frame save of the 1920x1280 72dpi image in a 1920x1080 storyline (spatial conform: Fill):
HTH