First, let me try to address this issue from different points of view.
First, this, in theory, is in the scope of museography as a discipline. This would take into account the flow of people, the type of pieces the gallery is showing, the space available. But it is pretty hard to find information about this. So let me go to the next best thing I have: Viewing distances from tv manufacturers.
It seems to be some consensus on defining the viewing distance forming a 40° angle of view to the base of the monitor. I am assuming most of them are 16:9 proportion.
This results in a very easy operation. Take the diagonal and multiply by 1.2x. For a 75 inch monitor, you get 90 inches in viewing distance.
But another manufacturer gives some other relation. Multiply the height of the monitor by 1.5x. After some calculations, this gives the viewing angle of 62° on the base.
As this starts to be math-consuming let me post a table.
In short, this is a table of viewing distance vs size of a 16:9 image. In orange we have dimensions and in green we have proportions.
But let's think we can take some numbers from here and apply them to any other 2-dimensional image.
The range is between 1.37 and .84 on the longer side, the base of the image.
As I am a bit lazy, I always round the "recommended" viewing distance to the largest size of the image (within certain proportions). If your viewer will be at let's say 60cm or two feet, keep the longer side that size.
These viewing distances vary when it is a public space. Normally people tend to view the pieces from a greater distance, fearing blocking the view for other people. But also there are people trying to peep closer to a pice.
Impact of the pice
An exhibition is an experience, so "impact" is a keyword. Watching some monumental photography is always interesting. The limit on which size would I print is a more technical one. Can the original image hold the print resolution?
Depending on how sharp the motive of the photo is, you can notice fewer or more pixels. Imho it is hard to notice a pixel below 100PPI, but in some cases, for example on a portrait, the eyelashes or hair can show them more than on the chin. Someone could think some resampling is not allowed, but I think a 200% resample is applicable to hide these pixels, making the print resolution 200PPI. You probably will see the droplets of ink before distinguishing one pixel.
A 24Mpx image can hold 60 inches on the long side. If the image is worth it of course. 30 inches if the print quality is exceptional.
But remember, as an exhibition is an experience, I am sure you could print some photos to cover a wall.
The photo as an artwork.
If you intend to sell the photos, probably you do not need to go that big, but only up to a certain size.