Side notes

I find the use of “grand” rather than “gros” to convey “big” a bit out of tone for what the novel is. Perhaps there is some sort of taboo related to broadly and liberally using “gros” in written (and maybe even oral) French, I’m not sure, but in any case I wouldn’t refrain from using it here.

Also, I find “chose” a bit ugly in this case. It may be strictly personal taste, but I feel “truc” makes the whole sentence flow better.

…and then some

I’d propose “en avoir de reste”. RESTE on the TLFi.

Plus gros que le plus gros truc de tous les temps et il y en a de reste. Beaucoup plus gros que ça en fait, vraiment incroyablement immense, des dimensions absolument stupéfiantes, un véritable moment « houla, ça c’est gros »¹.

Bigger than the biggest thing ever and then some. Much bigger than that in fact, really amazingly immense, a totally stunning size, real ‘wow, that’s big’ time.

¹ _{This part is imperfect for failing to infill the quoted text within the statement, but nouns after adjectives are not super commonly used in French and that’s as good as I could make it. Since it’s not related to the question, I’ll live with the consequences of that poor choice.}