In this usage, "either" is an adverb, and is used for emphasis after a negative statement.
In your example, I would not put a comma on each side of "either"; rather, I would say:
1. But he's fought at a much lower level. Not the biggest heavyweight in the world, either: at 6ft 2ins, he wears opponents down more than he bangs them out...
Or you could use a dash instead of the colon.
I would paraphrase "either" here as "moreover", or perhaps "I would stress".
All the best,