[dragonwriter]

> Where do you read numbers can be arbitrarily large?

RFC 8259, Section 6, specifies unlimited number of digits for the integer and fraction parts, but allows implementations to have size and precision constraints.

> I thought it was limited to javascript numbers.

It suggest that IEEE754 binary64, because it is widely available and supported, should generally be safely usable (which happens to be exactly JavaScript numbers) but it does not limit JSON numbers to that range/precision.