It seems like there is no maximum limit to a BigInt as per spec, which makes sense considering BigInts are supposed to be arbitrary-precision integers, whose “digits of precision are limited only by the available memory of the host system”.
As for v8 specifically, according to this article on the v8 blog, the precision of BigInts are “arbitrary up to an implementation-defined limit”. Unfortunately, I couldn’t find any further information on how the limit is determined. Maybe someone else would be able to shed light on this based on these v8 BigInt implementation notes?
That said, based on the aforementioned articles, there doesn’t seem to be a specific maximum value/size for a BigInt. Rather, it is likely determined based on the available memory on the system in some way.