I found this old-looking pic of the Star Tours AT-AT. You can get an idea of the size based on that worker down near the feet (standing next to the ladder in a helmet).
It does look like a forced-perspective job. The proportions seem to get a little smaller as it goes up. IMO the head & torso look about 1/3rd-scale and it's a bit bigger down at the feet. The knee joint is obviously higher up on the leg too.
I think this is enough for a mainstream theme park. Sure, bigger would be cooler, but whatever. Gotta draw the line somewhere. The resources & engineering needed to build the thing would probably go up exponentially as the size gets bigger.
View attachment 714338
The original "theme" of the Disney MGM Studios / Star Tours attraction was different from the Disneyland version. It was supposed to replicate a live movie set; the the AT-AT was a movie prop / facade, and the posterior/ backside metal supports and struts are clearly visible when you walk underneath. The back is unfinished, on purpose. When you first walked into (the original) Star Tours, there were plywood type set consrtuction pieces and directors chairs, to look like you were entering a studio "set" of the Star Wars film.
This fit in with the overall park theme in the 1990s, that the theme park was an actual functioning film and TV studio, with real sets and shows to visit. While some TV production, post-production, and animation was done at (then) Disney/MGM studios, that time has long gone. The animation and backstage tours are now a thing of the past. Live production (for the most part) and animation are long gone.
Since then, the story to the Star Tours attraction has changed, and the "hot set" story line has been removed. However, the AT-AT was never "finished" or updated. It has been refurbished / the outer skin removed, but it was never completed to look like an "actual" walker.
And the Endor Ewok village trees were false fronts as well, again pushing the "this is a movie set" theme.
Still impressive. But about 50% the size of a "real" AT-AT.
Here are some pics that show the "live set" theme to the attraction.
Also, the walker guns used to "fire" (water/vapor burst). See Video below: