Also shielding can come into play depending on the situation. Its hard to find 20m cables under 20 to 25. The 20 and 15m cables from PC to TV both give a picture slightly bigger than the TV, yet move PC and use short cable its fine, no idea why. Neither will work from my handycam to my PC but short El cheapo cables do.
The HDMI standard has ratings, there are sites that have done tests, but PCs tend to output a signal far stronger than required by the standard so tend to be fine except in special circumstances