Depends on which version of AC you're talking about, size-wise. In some games, human beings stood up to a bit over ankle sized, and tanks and the like being sizeable threats to an AC. I personally hit my peak during the Armored Core 3+ Era, with crest, kisaragi and mirage being the corporations permenantly engraved into my brain. In that series, a tank was roughly the size of a single AC's foot.
The thing a lot of you seem to be forgetting, is that with a single series of a show/game/whatever of anything, the japanese like to make completely alternate universes that are more or less exactly the same. CASE AND POINT: Tenchi Muyo and it's sister series had more than three seperate series that were in no way related, but had all the same characters.
EDIT: and the same basic plot, goddamn