There are great hopes that the direct methanol fuel cell (DMFC) will power portable electronic devices, such as mobile phones, laptop computers, and video cameras. To inform designers performing preliminary design studies on such portable electronic devices, in this study we developed design data on the power generation performance of small DMFCs, showing how performance depends on operating temperature and flow rates of air and methanol-water solution. Utilizing commercially-available parts, we constructed a small DMFC with an active area of 4cm2. We then systematically investigated power generation performance as a function of operating temperature and flow rates of methanol-water solution and air, by controlling the temperatures of the cell, the bubbler and the piping, and the flow rates. We clarified the optimum operating temperature and flow rates for maximum power performance, and the optimum excess air ratio from the relationship between the excess air ratio and the maximum power density in the small DMFC. One anomaly in our experiment was an occasional sharp drop in the cell voltage of the small DMFC. To investigate this sharp drop in cell voltage, we produced a visualization cell, and observed the flow fields of separators in the anode and cathode sides. From our observation of the flow fields of separators, we also clarified constraints in the design and use of small DMFCs.