To answer the questions you asked, I've had good luck with DH Labs cables:
https://silversonic.com/products/digital-cables/And way back in the day when I first saw that coax ought to be at least 4' long and optimally 8' I tried a 3' cable and an 8' cable (both cheap from Amazon) and couldn't hear any difference and I've gone with 1 meter ever since. So I'll say you are OK with 1 meter.
Now for the question you didn't ask, so this is JMO/FWIW and all that. I'm a Denafrips "true believer" myself and have seen many online discussions about adding a DDC and most people would say you are better off moving up to a better DAC, i.e. at least a Pontus rather than adding a DDC to the Ares. This is exactly because the real power of the DDC is realized with the I2S connection to the DAC. In fact I think a lot of people would argue that going USB -> DDC and then coax -> DAC is a step in the wrong direction and you might be better off without the DDC at all. I fully understand how easy it is to spend other peoples money but in your situation I would look hard to see if there is any way you can get a Pontus, I think you will be impressed at the difference.