Presumably he means without using any technology to help communication
Actually it was 100.
That very poorly written article, lacking depth, doesn’t say at what point in time in the history of Rome it is referring to. As has been noted before, the number of people in each level was reduced, probably due to reduction in the decuria.
Welcome to the Straight Dope, by the way, and thank-you for your contribution. Others with more knowledge will be along shortly to take up the discussion, I’m sure. 
Even that article says “about 100 men.”
Twice!
You could call yourself an “Eightyper Centurion”?
(Seriously- this thread is much more interesting than I’d expected,)
Cause no one wanted to be called an octogintarion.
The company size point is a good one - and is close enough to the lower bound to Dunbar’s number to make me think there’s some sort of connection there.
Alessan, are you replying directly here to AB?
[off-topic] I found it very easy to memorize the names of officer ranks in Thai. The same names are used for Army, Air Force, Navy, Police and Immigration ranks and can be roughly translated as:
4-star General - Mr. Power, 1st
3-star General - Mr. Power, 2nd
2-star General - Mr. Power, 3rd
1-star General - Mr. Power, 4th
Colonel - Mr. Thousand, 1st
Lt. Colonel - Mr. Thousand, 2nd
Major - Mr. Thousand, 3rd
Captain - Mr. Hundred, 1st
Lieutenant - Mr. Hundred, 2nd
2nd Lieut. - Mr. Hundred, 3rd
Sergeant - Mr. Ten, though I’ve never tried to memorize the detailed gradations among NCO ranks.
That would be 81.
(Sorry, this is SDMB. If not me, somebody else…)
I dunno. It was five years ago, so your guess is as good as mine.
I’m afraid I don’t have a cite handy, but I know I’ve read that humans typically max out somewhere around 100 with the number of people they can have a working knowledge of–maybe the captain doesn’t know everything about Private Jones, but he knows Jones is hotheaded, a deadly shot, and a very slow runner; and that this is indeed why we find humans organizing into groups of around 100, eg the infantry company. Bigger than 100 and it starts to be “who is that again?” However without a cite I can’t say whether this is credible or sort of folk sociology.
Dagnabbit!
Another question might be why centuries in Roman civilian life - for voting purposes - could often contain several thousand people. Answer: They started off with about 100 in the early Republic and ‘just growed’.
Centuries in the military ‘just shrank’. But in general, nominal numbers rarely reflect realities in military units, for many practical reasons.
It’s not a good idea to get too hung up on neat and tidy numbers and organisation in the Roman army. It always varied - just like Roman armour, shields, weapons, and tactics. If you look at actual Roman depictions of soldiers, most of them don’t look much like modern re-enactors.