The present invention provides methods for manufacturing microstructured optical fibers having an arbitrary core size and shape. According to one embodiment of the invention, a method of fabricating a photonic band gap fiber includes the steps of forming an assembly of stacked elongate elements, the assembly including a first set of elongate elements, the first set of elongate elements defining and surrounding a core volume, and a second set of elongate elements surrounding the first set of elongate elements, wherein the core volume defined by the first set of elongate elements has a shape that is not essentially an integer multiple of the external shape of the elongate elements of the second set of elongate elements; including the assembly in a photonic band gap fiber preform; and drawing the photonic band gap fiber preform into the photonic band gap fiber.