You're correct about it being a single use ability, and yes, it seems other people have been experiencing that lately (going by other threads like this, and iirc one of my guildmates mentioning it). I had the same problem against Starkiller one of the previous rounds this GAC. The moment he used Size Means Nothing, he gained a bonus turn and used it again, completely wiped me out. The only thing I can think of is that it's not WAI when used with the "Attackers attack once again on a critical hit" DC. But if you were using a different, if any, DC, then I can't explain it. In any case, it's definitely a bug.