That's roughly 19000 x 24000 pixels. Nobody needs that much. You can safely drop ppi to 100, and even that may be overkill. Generally, people always vastly overestimate resolution requirements.
Even if you did need that, those specs aren't nearly enough. 8GB RAM won't even get you started. For this you should have at least 32, and a very fast scratch disk with at least 500GB free (because that's where most of the load will be, irrespective of RAM). Understand that this is a huge file.
Here's the deal. You want people to be able to walk right up to this. But they won't do it! The brain simply refuses to do that, because it wants to take in a reasonable part of the whole. Otherwise you can't make any sense of what you see. It's like using a microscope on a magazine page.
As you walk a few steps away, resolution requirements drop really fast. At 60cm / 2 feet, anything above 100ppi will be so crystal sharp you can cut yourself on it. A meter away, 72 will do splendidly.
You have given me plenty to think about, and I think I can make this project work based on your observations.
However, in this case I do want viewers to see the whole and be drawn into the detail enough to get close — though the 2 ft. you mention will likely be close enough. Think of the 19th Century Hudson River School paintings, which commonly showed great swaths of nature with small human details scattered about. Frederick Church’s “Heart of the Andes” below is a case in point. If you zoom in on even this small jpg you can see a village in the distance, a woman at a cemetery, and a tropical bird perched in a tree on the left. Viewers always walk up to these grand old paintings to take in the details.
That said, Church’s dimensions in this painting are 5.5 ft x 10 ft — notably smaller than in my opus — and the scene doesn’t suffer from the more modest size. I think I would do well to reconsider my ambitions. That combined with your insights about resolution should get me out of the woods.
I appreciate the help.
I understand. I still think 100ppi will be good enough. That's still a lot of pixels, and it should still look very impressive..
Is this a digital painting, or based on photos?
Right. Good luck, looks like a spectacular project.
What I was going to say about photos, is that they are usually a bit soft as they come out of the camera - even with all the pixels of a high-end modern DSLR. So the most important thing you can do for critical up-close viewing is a bit of careful sharpening. This will often make a much bigger difference than higher pixel count.
Of course, a brush stroke in Photoshop can be as sharp as you want.