This function defines a box shape for your fixture. It takes the half width and height as the physics world uses this value far more than whole width/height values to determine things like collisions.
physics_fixture_set_box_shape(fixture, halfWidth, halfHeight)
|fixture||the index of the fixture|
|halfWidth||the half width of the box|
|halfHeight||the half height of the box|
The code above will apply a box shape to the fixture indexed in the variable "fix_Border" with a width of the room and a height of 20 pixels.