What is needed in order to produce good quality photographic wallpaper for a smallish room?

To capture enough pixels to meet your needs, the best option is a mosaic. In the computer, a panoramic can be made from a row of images. A mosaic is made from multiple rows.

To do this with high precision is an advanced photo technique. A special tripod head that can move the camera precisely is required.

Unless you have money for the special tripod head and time to learn this, you should hire someone.

http://esartprints.com/articles-reviews/33-basics-of-panoramic-mosaic-image-making

If this link breaks, google "panoramic mosaic".

To print something this large, search the internet for companies that specialize in this.

Consider the size of the area to which you are projecting or printing the image to. I am going to assume you are printing. We should probably note that this may not work well if you are standing close to the wall to view it. Large images over large areas work better if you stand back a little. Additionally, the printed image will appear skewed when viewed if you stand very close to it. If you have your desk against this wall, it may look disorienting. But if the image is along another wall, or a little bit farther from your desk, this could look ok.

Most printed photos like 4x6, 5x7, 8x10 are at a resolution of 300 dpi. So if you achieve 300 dpi, your wall will appear to have a photo grade quality to it. Of course, this is extremely hard to do. So you will have to live with something less than 300 dpi. But we can try to determine what image you will need.

Methods

Here are a couple ideas. Mattman suggested a moasic stitch technique, which is the same idea as #1:

1. Using overlapping digital images of a scene, you can stitch them together to produce one large image. If you have a recent smartphone where that has a panorama mode, you use it by swinging the camera across a scene. This is basically shooting several photos and stitching them together to make a wider image. You can rent a camera and lens to help take better images. You can even rent a panoramic head to help you position the angle for the photos. There are computer software and plugins you can use to auto stitch images together. You could also try doing it manually. If you are only interested in this one thing, you might be able hire someone to stitch it together for you.

2. Using an expensive camera you can rent, it may be possible for the sensor to shift, and collect more image data to produce a much higher quality image than it normally could. Sensor shift images usually take 1 second or more to be produced, so it is best not to have motion in it. You said you want an image of a harbor. Boats bobbing or water moving may or may not be ok with this. I am sure someone has tried already, and you can check their results. The good thing about this is that you can just rent the camera and lens, frame your scene, and snap once. The resulting image requires little to no post-processing. You may not need to hire anyone for this.

Determine the image size you need

We can do a little math to determine what image size you should try to produce. If your wall is 6ft wide, that is 72in, which at 300dpi is 21600 pixels. A standard ceiling height may be 9ft, which is 108 in, which is 32400 pixels. 21600 x 32400 = 699,840,000 which is 699 megapixels. This is unrealistic. Large prints are never 300 dpi. They may be 100 or even 80 dpi. Just remember that the lower the dpi, the more pixelated or grainy a print will look as you get closer to it. But this doesn't always have to be a bad thing. In some settings, this is an intentional art form, like halftone. At 100 dpi, you would need 77,760,000 or 77.7 megapixels. This is actually becoming possible using method 2. A Sony a7riv camera's native resolution is 61 megapixels, and the pixel shift high-resolution mode can produce 240-megapixel images.

You said you wanted a minimum distance of 2 feet away. What you could do to help test acceptability is to check if your printer software can adjust dpi. Print some full page (high quality photos) at different dpi and tape them to the wall. You can then stand near them and see how to feel about the dpi differences.

I am going to recommend method 2, but there is no reason why you can't also take more images to attempt to stitch them at the same time. You can even rent a panoramic head to assist with taking the photos for method 1.

Rent equiptment

You can rent equipment shipped to your address from BorrowLenses or LensRentals. You may also have local shops which can rent you equipment. But if you rent, and you are unfamiliar with dslr or mirrorless cameras, you may want to learn how to use them. It is also possible that you may find photographers in your area that you can commission to produce and process the image for you.

I recommend going with a high resolution, pixel shift capable digital camera, like the Sony a7r iv (An alternative is a Panasonic full-frame camera, but that has fewer lens options for rent at a decent price at this time). For wide landscapes, wide focal length full-frame prime lenses like a 24mm or even 18mm should be good. You will also need a decent tripod, camera battery, maybe even sd cards (if you don't buy them yourself) and you will need a card reader or camera usb cable to transfer these to a computer of course.

If you also want to do option 1 at the same time, you will need a standard or slightly telephoto lens. 50mm, or 85mm should be good. If you plan to try both methods, you could rent one quality zoom lens like the Sony FE 24-70 f2.8 GM, instead of several lenses.

Find a printer

Once you have a high-quality image, you could resort to printing them out yourself and arranging them, or you could find a commercial printer that may assist you with the size you want. Actually, if you plan to solicit print services, I would talk to them about what they can print and what images they would need before doing anything else.

I hope this gets you on the right track.