This is a great question...it is very important to select an appropriate print size for your target room, as there will be the need to stand back from your photomosaic to see the target source image.
The larger the the mosaic print size the further you need to stand. I have created several mosaics and found a a good rule of thumb is for every inch wide/high, you will need to stand 1/3 foot. In other words, for a 24x36 inch mosaic you will need to stand about 13 feet away (36/3 = 13) to see the target clear. This is not an absolute rule, as cell size and cell count will contribute to this estimation. Hopefully this makes sense and is somewhat helpful.
I always like to have a mosaic as large as possible for the room it is intended!
