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?