Creation of an array which length depens on variables not possible?

[TheDGuy via Digitalmars-d]

Why is this not possible?

                GtkAllocation size;
                widget.getAllocation(size);
                this.width = size.width;
                this.height = size.height;

                int[size.width*size.height*3+1] arr;
                this.accumulator = arr;

nor this:

                GtkAllocation size;
                widget.getAllocation(size);
                this.width = size.width;
                this.height = size.height;

                int[this.width*this.height*3+1] arr;
                this.accumulator = arr;

in the constructor?