I recently got my hands on a very nice MIDI file modifier program 
that adds note spacing in milliseconds to MIDI files that are 
destined to play on digital player pianos like the Yamaha Disklavier 
or QRS PNOmation, etc.

Can someone tell me how many milliseconds of space there should 
be between notes and what the minimum note length should be in 
milliseconds as well (to prevent notes of zero length)?

The program was made by this guy: https://musescore.com/user/19465036 
Here's a Google drive link to download it: https://tinyurl.com/3y4kzzrf 

Piotr Barcz