How big? Very Big, Big, Normaly, Small, Very Small..?

According to some in game detail, they outnumbered BoS at some fight 20:1.

NCR had, New Reno, Shady sands, Redding, Junktown, Vault city, vault 13, gecko, the hub, san fransisco, Modoc, the den, Necropolis, Boneyard ( LA ) by the end of FO2.
Some may have been in worse shape than the others, but by and large were quite impressive in scale even for the engine at the time.
Years of mostly stable politics would lead to strong communities rich in numbers.

So I would put its army at very large.

NCR literally controls nearly all of California and the entrance to the Mojave area(Mojave Outpost). So in Post-War standards, it's freakin huge.

My NCR army is 7in big.

VERY LARGE. they control the whole state of california.

some of Oregon and Parts of Nevada, including New Reno

The figure I hear for the population of the NCR is ~700,000 people......thier Army probably is around 20,000, possibly as many as 50,000 since they are using conscription.

That was NCRs population before they gained total control of California and 40 years before New Vegas so their population coud be well into the millions maybe 2-5 million.

Relatively large. By that I mean they likely have a very large military compared to other military-like groups. Perhaps easily 10% of their population.

This is their extent, IMO.

Well, going on what we see in the game - very tiny, maybe 150 tops..

But going on canon, very big, 50,000 maybe? but if pushed I'm probably sure they could put 100,000 on the field

The only places we know for certain are part of the NCR are Shady Sands, Junktown, Boneyard, Maxson, The Hub, and Dayglow. We also know that in Fallout 2 they had 5 states, and in New Vegas it's said that they want to annex Vegas as its 6th state. So they did not get any additional states between FO2 and FNV, although some additional towns might have joined as part of the existing states, I suppose. But I doubt any major ones did.

They do have territories and some states expanded.

40.000 troopers who just pissed their pants in terror of a radroach and around a hundred rangers.

....

This is why I support Legion.

New Reno is part of the NCR by New Vegas. This is because one of the Fallout 2 endings is that the Bishop family, led by the Chosen One's bastard child, will take control and New Reno will join the NCR and become a tourist destination. This is the canon ending if you speak to Bruce Isaac in Novac. It's possible that New Reno and other areas are administered at territories. The NCR is partially modeled on the United States, and every time the US expanded they weren't like, 'oh cool, add another star to the flag,' there's a process. Also they'd have some funky-shaped borders to not include Necropolis.

Maybe, as posted by others Reno is mentioned as NCR, Ghoul rangers would sugest either Necropolis or Gecko are in imo, if not both.
Either that or at least have support / ties from those city states.

NCR is a water backed economy caused I believe mentioned elsewhere due to BoS destroying the NCR's stockpiles.
If they had a stockpile it would come from some where, hence Redding.

If Reno is in and given Vault cities isolationism, I made an assumption, but it really only stood a chance with Reno allied or neutral.

San franisco, Jaun Cruise and the cult he belongs to either took power and left / destroyed theselves like a bunch of space suited ghouls.
Or the Shi took complete power and logically sided with the strongest state in an effort to keep peace in their thriving city ( imo ).