I'm pretty sure the point @
meleebrawler
was trying to make was that, despite Lucas's N-Air covering his entire body, it doesn't mean much because he's smaller than Mewtwo. Given Mewtwo's size, I'd say his N-Air's range definitely beats Lucas's. Basically, although being small is generally an advantage in Smash Bros., it helps Mewtwo's N-Air cover more area.
It's very misleading to use the term "windbox" to describe auto-link angles. "Windbox" implies that the attack generates a push or pull effect that moves an opponent into a particular position. Of course, this isn't the case with Mewtwo's N-Air. It uses auto-link angles to knock the opponent into position. You wouldn't say that Fox's F-Smash generates a windbox, would you? Of course not! It inflicts knockback. It's the same deal with Mewtwo's and Lucas's N-Airs. They just inflict knockback at a particular angle in order to link the hits together. Hence, the term "auto-link angle."
I mostly agree with you here, but I would say that Mewtwo's N-Air is easier to land than Lucas's. It covers more area, and I'm pretty sure that it's out for longer. Also, Lucas has his own problems. Namely, he doesn't have many safe options. All of his Smashes are unsafe on shield, his U-Smash and D-Smash are both unsafe on whiff, his grab is very unsafe on whiff, his recovery is very telegraphed when not within tether-range, and, if I remember correctly, all of his aerials, barring his N-Air, have noticeable landing lag. Of course, that doesn't mean he's a bad character, but it does explain why Sakurai felt the need to buff his N-Air, along with some other areas.
Why? Going off what you're saying, having Lucas's D-Smash wouldn't help Mewtwo at all. Even with that shield push, Lucas's D-Smash is unsafe on shield. Once your shield is struck, if you drop it immediately, you can run in and punish Lucas without worrying about getting hit. It's very unsafe on shield. If I remember correctly, Mewtwo's D-Smash has little enough ending lag and inflicts enough to shield push to keep him safe. At the very least, it doesn't leave him as exposed as Lucas's does. Barring range, I'm pretty sure Mewtwo's D-Smash outclasses Lucas's in most areas.
I've found that, along with neutral spam (which is an admittedly questionable decision) and attacking recovering opponents (which requires god-like timing), Mewtwo's D-Smash can also be used to punish opponents who try to roll in on you, but misjudge the distance and wind up in front of you. They have to be within a certain range, but, when they are within that zone, it's easily your best punish. Also, certain characters, such as Pikachu, Jigglypuff, Kirby, Fox, and even Peach, having landing lag animations that put their hurtbox too close to the ground to be hit by Disable. In situations where said characters poorly space aerials that induce such animations, Mewtwo's D-Smash is also your best punish, provided they fall within the aforementioned zone. Granted, D-Smash is a rather situational move, but it does have its uses.