re: Yeah, I made another mistake
Optical efficiencies don't scale linearly. More to the point: a smaller lens wouldn't work. By some fluke I believe I hit the magic number with my original 1-Terawatt Fresnel lens with a 505-meter diameter. This is very close to the size required, I think. But before I make any more estimates, I'm going to do a serious analysis. So I'll re-post my idea when I'm done.
Until then, you can ponder this image which may actually be a somewhat realistic representation.