Recommendation for Digital Coax

Also I meant to ask is it true that it should be either less than a foot or more than 6 feet?

Yes, 1M is where to start but not sure as important with a streamer but with older transport with higher jitter it is not recommended to go shorter. There’s a few Nordost Silver Shadow coax on here which is a really nice cable for the money. I use one and very happy with.

1 meter is not a good recommendation for a digital coax cable as it will always have problems and compromised high frequency resolution.  I always use and recommend 2 meter, but many say 1.5 meter.

DH Labs D-750 is a good base to start from.  From there, check out Acoustic Zen.