Not known
Rockstar has released two trailers, about seventy screenshots and a set of location pages naming five regions of the state. Nothing in any of it states a size. Neither does Take-Two’s investor material, which has had every commercial reason to boast about scale and has instead talked about dates, editions and pre-orders.
That absence is the finding. A number missing from all of the material that would carry it, in a campaign this long and this deliberate, is not an oversight waiting to be filled in from outside.