The ultimate goal of our study is to determine the optimal size of pen-input boxes on handheld devices such as Personal Digital Assistants (PDAs) into which users can most efficiently write alphanumeric and non-alphanumeric characters. We define an optimal size as including the following characteristics: high performance and high subjective ratings (e.g. including ease of writing and minimum degree of fatigue). Character input box sizes for handwriting on PDAs differ according to the software application for which they are to be used. PDA screens and their character input boxes are very small making character input relatively difficult. However, obvious limitations must be placed on the size of input boxes so that the information domain, for example, is not dwarfed. Therefore, it is essential to determine the optimal size of the character boxes of PDA screens. We attempt to experimentally determine the optimal size from two viewpoints: character kinds and character shapes, based on two experiments. The results, which were assessed by the rate of character recognition, the number of strokes protruding outside the character box, and also by subjective ratings, show that the optimal size of character boxes for inputting alphanumeric characters is approximately 1.09 x 1.66 - 1.44 x 1.44 cm, and for Hiragana & Katakana and Kanji & Kana the optimal size is 1.44 x 1.44 cm. These results are useful for designers to determine the size of handwritten character boxes in both recognition and non-recognition situations.