You can use unity make a quad and scale it to see what the width and height are in meters for your poster. Then go to blender make a new plane, and use those dimensions. Then subdivide it until you have a good density. When you make a new obi topology file, there are scaling parameters there you can use if you need.
For backfaces you'll want to add two materials to your object, and for one of its shaders set it to standard (backfaces).
For backfaces you'll want to add two materials to your object, and for one of its shaders set it to standard (backfaces).