I got a message from Cheyenne urging me to go immediately to New World Notes. I did... and I'm dumbfounded.
Ceri Quixote has created a 62-meter mesh avatar.
There are other reports. There's a video... watch it in 720p mode and full screen.
Shadows are enabled, and the positions are such as to rule out forced perspective.
Note also the quality of the avatar; giant avatars built from prims have improved over time, but at the joints it's obvious that they are built up from multiple prims. Look at Ms. Quixote's shoulders and abdomen; they are smooth and the body is a single unit.
So... we have an existence proof. (The NWN article says Ms. Quixote will be selling (somewhat shorter) giant avatars on SL Marketplace.) I think I will go IM Bart Seymour...
UPDATE: Be sure to read the comments on the NWN post. Ms. Quixote is not alone in creating giant mesh avatars, breathtaking as her work is. Indeed, if you have read past articles and comments here, you'll recall that Aliasi Stonebender has done so and mentioned it back in mid-February.