Why were the heavens mentioned along with the earth in the description of the width of Paradise as, "its width is the heavens and the earth"? And what is meant by "We will forget them" in His Almighty saying, "This Day We will forget them as they forgot the meeting of this Day of theirs"?

Paradise is described as having a width like the heavens and the earth, meaning its width is like the heavens and the earths combined, a similitude of vastness and grandeur, not a precise measurement of width.

As for Allah's forgetting of the disbelievers in His statement: "So this Day We shall forget them as they forgot the meeting of this Day of theirs," it means leaving them in torment just as they abandoned preparing for the Day of Judgment. This is an instance of verbal congruence (mushakalah lafdhiyyah) in the Arabic language, where the recompense is of the same kind as the deed. Allah knows best.