With the caveat that there are wide areas of the United States where population density is exceedingly low, no single fixed network service provider has a geographic footprint that covers “most” of the landmass.
Here is Comcast:
Here is AT&T:
Here is Verizon:
Here is CenturyLink:
Here is Charter Communications:
Of course, many will note that what really matters is not landmass but potential customer locations, such as homes and businesses. The Charter Communications network passes about 50 million homes, the number of potential customer locations it can sell to.
Verizon homes passed might number 27 million. Comcast has (can actually sell service to ) about 57 million homes passed.
AT&T’s fixed network represents perhaps 62 million U.S. homes passed. CenturyLink never reports its homes passed figures, but likely has 20-million or so consumer locations it can market services to.