Many handheld devices with stylus pens are available on the market, however, there have been few studies which examine the effects of the size of the stylus pen on user performance and subjective preferences for hand-held device interfaces. Two experiments were conducted to determine the most suitable dimensions (pen-length, pen-tip width and pen-width) for a stylus pen. In Experiment 1, five pen-lengths (7, 9, 11, 13, 15 cm) were evaluated. In Experiment 2, six combinations of three pen-tip widths (0.5, 1.0 and 1.5mm) and the two pen widths (4 and 7mm) were compared. In both experiments, subjects conducted pointing, steering and writing tasks on a PDA. The results were assessed in terms of user performance and subjective evaluations for all three pointing, steering and writing tasks. We determined that the most suitable pen dimensions were 11 cm for length, 0.5 mm for tip width, and 7mm for pen width.