Help On High Quality Limited Edition Prints (ppi)

I have created several new works of art that I want to sell as limited edition prints. My question is what size should they be? I wanna do around 18x24 prints. Maybe even larger, but the ppi for the image I wish to make an 18x24 is 162. Is that a good number? I know the lower the ppi, the less appealing the piece looks, but one must also factor in the viewing distance. Can anyone help me out please?