A few years ago I created a life size heavy duty coffin and used it in my halloween displays. I wish I could give you specific dimensions, but to be used correctly , meaning to have someone in it to scare people, it must be big enough to fit the person you want to hang out inside of it. Mine is over 6 1/2 feet tall. All I did was use a piece of 3/4 inch plywood. Basically start by measuring the person who is going in it.  Make it taller so they aren't squished and hitting their head. Add a few inches at least. After all, you never know what shoes they may wear. 

Then measure how wide their shoulders are and how far down from their head the shoulders are.  Add the few inches to that height.  Start at one of the 4 foot ends of the plywood. measure down the distance to the shoulders. That's your starting point. Measure the width for the shoulders, keeping in mind the need for moveable space.  Make sure that the line you space for the shoulders is square to the 8 foot edge of the plywood. Mark the halfway point and draw a line from the top to the bottom at the width of the halfway point. From the center line you just drew, decide how wide you want the top. Cut that number in half and measure each side of the line at the 4 foot side that width. Use a straight edge to connect the edge of the top to the edge of the shoulder line. Then using the height you determined at the beginning, measure down from the 4 foot end and mark the center line where you want it to stop. Determine the width for the bottom, then use a straight edge to make the outline from the foot to the shoulder. Cut it out. That's your base. Once that's done, determine how deep you need the coffin to be and cut strips of the plywood that wide. Cut them to your lengths of each section and screw it all together. Paint it black. Have your person wear all black inside and create something away from the coffin to distract visitors so they don't notice the person in the coffin and have the person in the coffin reach out and grab the visitors. If was a ton of fun when we did this. I still use the coffin as part of my halloween yard decor. I also make a top for it, and attached an antique looking cross on the top. It's a halloween decoration, it doesn't have to be perfect and I'll warn you that if you make this to hold a 6' 6" man, it will be very heavy!