Let me alter the perspective, so to speak, LOL! I wish to print large. So I'd like to choose sensor-lens combinations to deliver 16x20 or 32x40 or even 64x80 prints, viewable at 12" as "perfect".
When I got access to a 24" / 60cm printer 3 years ago, I had to deal with this particular question. I found it easier to answer when making abstraction of the camera lens at first.
Even if I normally measure dimensions in metric units, I found that it is easier to use inches in that particular case, because everybody is used to resolution being given in dot per inches (dpi).
What is the needed resolution for large prints in dpi? For small photographs or generally continuous tone images about 300 dpi is the maximum needed, as most humans cannot see anything finer resolved without the aid of a magnifier. Interestingly, this is also the resolution of fine art magazines and retina displays. So, we will use the value of 300 dpi as a maximum limit for small prints.
For large prints, that value is relaxed a bit. My feeling is that a value of about 150 dpi sufficient. The prints still stay apparently sharp when examined close up. Note that while about 300 dpi is relatively well defined as an upper bound (it comes from the characteristics of the human vision), that value is more fuzzy. It depends on the particular picture printed and one's particular taste as to what constitutes a "sharp print". Still, 150 dpi is about in the center of what is used for large fine art prints that look quite sharp when examined up close. For comparison: computer screens are about 80-120 dpi, large 4K TVs are about the same.
Taking the value of 150 dpi, what camera resolution do we need for our prints?
The first question to be asked is: what size can we print? Fine art inkjet printers and papers are available in the following approximative width: 17" (40cm), 24" (60cm), 44" (110cm) and 60" (150cm). For bigger sizes, we would need a C-print, the Océ LightJet500XL printer is 76" (2m) wide (think Andreas Gursky). These machines are not manufactured any more, the only remaining manufacturer is ZBE Chromira, and their wider machine is 50". Larger printer sizes are available, but not in this kind of quality. That gives us the maximal height of a landscape print, the other dimension is given by the aspect ratio. We will choose 3x4 as an aspect ratio.
The number of pixels necessary is thus given by the following formula: (150 x w)^2 x 4 /3. For the available widths:
17: 8.5 mpix
24: 17 mpix
44: 58 mpix
60: 108 mpix
76: 173 mpix
But please keep in mind that these figures are approximative and derived from our 150 dpi value. And because that value is squared in the formula, small modifications in the dpi value will result in large modifications in the mpix value. Conversely, having a bit less or a bit more mpix, will only lower or increase the dpi value a little bit. taking for example the 24" print, at what dpi can we print it with various cameras? The formula here is:
(√(mpix x 3 /4)) / 24, so:
12 mpix: 125 dpi
24 mpix: 176 dpi
34 mpix: 210 dpi
and, in my experience, the difference between a 150 dpi and a 210 dpi print is barely noticeable. 120 dpi, on the other hand, will be problematic for a landscape with fine details but may be ok for a soft portrait.
Coming back to cameras and lenses, we see that most 24x36 cameras will be sufficient for 24" prints. The question is: are they sufficient for the next size up: 44"?
Finding out the resolution given on the prints by the highest resolution cameras: the D800 and A7r is quite easy. The frame size of the sensor is 7360 × 4912, so we just divide 4912 by 44 to find out the resulting dpi: 112 dpi. If we need to print at 150 dpi with that camera, the maximum print height will be about 32" (85cm).
What about the lenses on the camera, then? Well, because of the vast amount of pixel peepers on the Internet, answering that question is relatively easy: printing at the maximum size allowed by the sensor resolution and expecting a sharp print examined close is the same as examining a 1:1 detail of an image on-screen (the computer screen dpi resolutions are comparable to average prints, remember?). And the answer is: there are very few 24x36 lenses which are perfectly sharp corner to corner on the D800 / A7r especially if you need a wide-angle lens. So here again the practical answer may be to limit yourself to 24" prints. On medium format, the answer may be different since there are many lenses which are sharp corner to corner on the highest resolution MF cameras, including wide angle lenses.