Exactly how large is the NCR's sphere of influence?

[Mutant Screg]

Obviously they have most of California, perhaps a chunk of Baja California and a decent amount of territory further east into the Mojave, but where else have they spread? I imagine that they would at least try to head up north a little bit into Oregon and perhaps even Washington. I like to think they have an outpost or two to scout out the territory for either threats, resources, or simply just to chart out the area for future endeavors.

 

[Knight In Leather Armor]

Nowhere else besides the West Coast, I believe. There is no mention of the NCR in 4 or 3, so obviously the East has no idea who they are.

 

[Jogre]

Nowhere else besides the West Coast, I believe. There is no mention of the NCR in 4 or 3, so obviously the East has no idea who they are.

3 and 4 are written by Bethesda, so they aren't good citations for lore.

 

[The_Time_Master]

Nowhere else besides the West Coast, I believe. There is no mention of the NCR in 4 or 3, so obviously the East has no idea who they are.

Radio: "All five states have now signed on, which means that as of this moment, we are all citizens of the New California Republic"

From the Dangerous Minds (walking in Kellogg's memories) quest in Fallout 4 (a split second before the captioning changes to "Mom knew how it was...."):
0 minutes 59 seconds into the video

 

[cordelionreaver]

Nowhere else besides the West Coast, I believe. There is no mention of the NCR in 4 or 3, so obviously the East has no idea who they are.

The NCR and Shady Sands are mentioned in note about the Mysterious Stranger in Fallout 4.

http://fallout.wikia.com/wiki/The_Mysterious_Stranger_(note)

So it seems Nick knows about the NCR.

 

[naossano]

As big as the plot demands.

 

[Knight In Leather Armor]

The NCR and Shady Sands are mentioned in note about the Mysterious Stranger in Fallout 4.

http://fallout.wikia.com/wiki/The_Mysterious_Stranger_(note)

So it seems Nick knows about the NCR.

Oh, right. I forgot Kellog'a Frosted Flakes and Nick mentions the NCR. However they still have no presence in the Commonwealth.

 

[mithrap]

Considering that the NCR had no idea what was east of its own borders for most of its existence, and had to send foot scouts to discover Hoover Dam, I really don't think that their influence is anywhere to be found beyond the main cities of California. The presence of NCR soldiers in the Mojave already stirs up rumors of "the NCR spreading itself too large", so I always understood that this was their first real expansion campaign outside of California (and its countryside is still dangerous and plagued with brigandage, not really a sign of an expanding empire with a stabilized heartland)
This is probably why they actually take the Legion's threat seriously, no matter how badly equipped they are. Caesar owns a HUGE territory, compared to the estimated NCR turf. And with that comes the demographics.