Upload & Sell: On
A few things to think about:
1. Enlargements fail when magnification begins to frustrate the mind's expectation for detail.
2. Number 1 above means that subject matter is important. Some subjects can be enlarged more than others. A portrait has terrific enlargement potential because we see people all the time, but our brains are attuned to features no smaller than hair and eyelashes. If skin pores don't show up in an enlargement, who cares? By contrast, a landscape including beach sand may start to get limited pretty fast, because the texture can be very fine grained, and is familiar to everyone. When that texture turns to mush, the expectation of detail is frustrated, and the enlargement suffers.
A corollary is that some subjects have been done to death by large-format film photographers, precisely because an 8x10 view camera is wonderful for recording textures. Take on some subject like bristle cone pine tree bark and you get in trouble pretty quick, because there are famous images you are unlikely to be able to match with your smaller-format camera.
3. Viewing distance, as others said.
4. Perfect technique maximizes enlargement potential. Steady tripod. Perfect light. The right shutter speed if it's a moving subject. Optimum exposure. Optimum aperture. Nail focus. Did I mention perfect light? The key to enlargement is delivering detail. Better light is the key to recording more detail.
5. Lens. We are only now beginning to see zoom lenses that can compete with the best primes for making big enlargements. My view is that even the new zooms don't yet compete and win. Good as the new zooms are, it seems to be too hard to make a zoom that reliably delivers uniform sharpness and contrast across the frame. For now, the reliable tools for maximum enlargement are the best Canon (or Nikon) and Zeiss prime lenses. Maybe some copies of the new zooms do it too. I haven't had one yet—still hoping.
6. Your camera sensor matters. Crop sensors will never be the best way to get a big enlargement. Whether full-frame sensors should be pushed to the pixel limit (thinking of the new Nikon D800e) is an interesting question that I would love to have an answer for. Would it be better if it were only 28MP? How about 42MP? I don't know. I do know that other things equal (they usually aren't), however many MP you are using, a cropped sensor is a worse way to deploy your pixels if you intend maximum enlargement. Tradeoffs with pixels and framing, especially for distant subjects, complicate the analysis, however.
7. Use the whole frame for your subject if you can. If you want big enlargements, needlessly throwing away pixels by cropping is foolishness.
8. A major factor in some images, especially the big sweeping landscapes that include distant subject matter, is atmospheric quality. Astronomers refer to a quality of the air they call "good seeing." If you are trying to image sharply any distant subject, you are going to need good seeing. That is not the same as the transparency of the air, and in fact maximum transparency rarely coincides with the best seeing. Instead, good seeing is related to steadiness in the air, and uniformity of the air mass. You can learn to recognize it. When there is good seeing, distant objects appear unnaturally close and detailed to your unassisted eye. Times like that are uncommon in most places (but relatively more common in some locations, such as Southern Florida), but intervals of good seeing are the time to make landscapes you can enlarge the most.
9. These days, selling big pictures seems to be easier than selling little pictures. So it may be worth notable trouble and expense to learn how to optimize your enlargements, and equip yourself to do it. But bear in mind that unless you are doing studio work, so many factors play into getting good results that you have to work at it steadily just to assure that you will be present and shooting when conditions all come together. A highly successful big enlargement of an outdoor scene is not something you expect to capture every day. Ansel Adams has been quoted as saying that if you can get 10 good images a year you are doing well. He was using large format cameras. It's harder with 35mm. Plan accordingly.
If you can make all that work for you simultaneously, my experience has been that you can sometimes get an image that pretty well withstands gallery inspection at a 45-inch enlargement. I'm using a 5D II, and Zeiss and Canon prime lenses.