I am doing a poster that is 68" x 47.25". The printer specs say Mechanical Proportion and Scale to be 1/4" = 1' and digital resolution is best at 600dpi.

My question is, what size should I set my document up as? Thanks in advance.

I took it as dividing the height and width by 12 to get the dimensions in feet, then dividing by 4, but this seems extremely small even though it is at 600 dpi.