The AT-XT is, actually something from past EU sources for the classic trilogy.
It's actually appeared in numerous comics and such. It was an early production sketch of the AT-ST used as a variant walker in the employ of the Imperial Armor divisions.
Possibly it has a different specialty, or simply is left over stock from the clone wars that the Empire still choose to keep in service.
As for its scale, it wouldn't vary in scale much from an AT-ST judging by pics I've seen of it, so if they made one, even to-scale (An approximate scale that is), it wouldn't be a large item at all.
AT-ST's are only over 8 meters tall. Not that big, especially when the height's all from the thin legs, and there's not much "body" to it. Same with the XT.
I'd take these just to use in classic dioramas then.