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I personally would not go over 6 ft. However, there are those rather expensive gold tipped HDMI cables, that are much longer. They more provide consistent resolution all the way through.

Not the same, but, at my old workplace, we installed a 50ft HDMI cable for a TV, with the said gold tips, and it worked fine. When we'd tested it with a cheaper 75ft cable, it was awful.

With most places having a return policy, maybe purchase one, and test it for the area in which you wish to use the Mac Mini...
 
There is no such thing as maximum length HDMI cable for <insert device here>. The HDMI cable itself has this property, not the device. HDMI can carry a wide range of resolutions at certain refresh rates. So the question to ask is: this specific cable length, what resolution and refresh rate has it been certified for? 2K@60Hz? 4K@30Hz? You get the point...
 
There is no such thing as maximum length HDMI cable for <insert device here>. The HDMI cable itself has this property, not the device. HDMI can carry a wide range of resolutions at certain refresh rates. So the question to ask is: this specific cable length, what resolution and refresh rate has it been certified for? 2K@60Hz? 4K@30Hz? You get the point...
Thinking of getting this:
 
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