Well, I haven't had the difficulty of being mistaken for a child in a very, very long time myself.  I'm 6' 2" and I'm the runt of the litter, most of the men in my family are 6'4" or better.

As to the job of the moderaters deciding what is and isn't a child for the purposes of a 3d rendering, that's one job I don't envy in the slightest.  Renderosity could get in a whole ton of trouble real quick if some overzealous legal type decided to make a huge issue out of a rendering being considered "child" pornography.

Granted, I've never heard of such a legal case but I'm quite sure that Renderosity would not like to be the first to try and defend themselves in court over something like that.  The punishments for such things are usually severe and the determination of what is and isn't legal in this regard is in a pretty big grey area overall, so quite frankly I can see why the moderators would tend to err on the side of extreme caution.

As to the issue of "short adults" in renderings, I've had my best luck with V3 and using the scale dial, scaling her down to about 70-80% and then doing some morphing.  If this doesn't make her "short" enough I just increase the scale on the other items in my render to compensate.   I dislike the proportions that using the "figure height" parameter gives me.

I've dabbled a bit with A3 I can't say as it's ever really been a favorite figure of mine.  Don't get me wrong, I've seen some really fabulous characters that were made for A3 by some of the various vendors here, but personally I've just never had the knack for it.  Every character I've ever tried to create using A3 has just never really turned out quite the way I wanted, I can never get a "realistic" look to an A3 character.  Something is always just a bit.. off.  Not sure if I can really explain it any better than that.