We demonstrate that laser peak pulse power can be employed to tune carbon single wall nanotube (SWNT) diameters. The production of SWNTs was investigated at room temperature and at 1200 degrees C. The diameters were shifted to smaller sizes in both cases as the pulse power was increased. SWNT size distributions and yields were studied with Raman spectroscopy and transmission electron microscopy. The evolution of the material quality with laser energy parameters offers insight in to SWNT formation mechanisms. These studies should aid in the development of methods for the rational control of SWNT growth.