Below are standard sizes and their recommended uses:
20" x 24" this is a personal size ideal for your bedroom or your work cubicle.
24" x 24" again, a personal size.
36" x 24" for teams and groups at workplaces, classrooms, and Dean's or Principal's office.
42" x 32" is the ideal wall-mounted size for offices and public places.
48" x 34" is for information that has to be read from a distance of 4'' and it has to be wall mounted.