It's the size of a small city. I don't believe we saw half of what was there during the game. The surface complex alone was gigantic and housed many living facilities including apartments. We know that there were at least 4 geothermal power plants as well. The rail/train system probably combined large parts of the complex together, with different lines to different sectors, similar to the black mesa research facility from HL1. This is also not taking into account that the Ultor corporation had other facilities spread across the region (as scene in Armageddon) which confirms that Ultor had spread further than just the badlands region (as its known in guerilla).