Is 6 feet tall too short for a heavyweight champion?

No, but reach is far more important than height is. And skill is very important, more important than height is.

 

Unless someone makes an exact clone of Mike Tyson (5'10 not 5'11 I've seen him in person)
or Evander (6'2) it's getting harder to see a HW champ being under 6'3.

Of course, it could happen but I see no one in the pipeline ATM

 

I do not think it is impossible for a 6 foot heavyweight to dominate the division in future, but it would take a wealth of physical attributes, mental sharpness and copious amounts of skill.

You need a 6 foot man that is large framed, brawny but not slow, somewhat punch resistant... and explosive as HELL!

Then give him a good boxing pedigree in the amateurs to build up his skill and killer instinct.

He'd have to be twice as good Povetkin lol

 

For sure in todays scene.