Aim First, we objectively quantified the handwriting size of Parkinson's disease (PD) patients. Second, we investigated the efficacy of the external visual cueing on Korean handwriting in PD. Methods We recruited 57 patients diagnosed with PD and 37 community‐dwelling normal older adults. All participants were asked to write a target word on a blank sheet of paper. For investigating the external visual cueing, 24 PD patients wrote the target word in square grids. The size of each syllable of the word was measured using a public domain software program. In order to measure the area of a syllable, the upper, lower, left and right margins of the syllable were outlined with a rectangle. The area of selection in square pixels or in calibrated square units was used to spatially calibrate the image automatically. Results The PD group showed significantly smaller writing size than the normal group in all syllables of the free writing. The writing size in the square grids of the PD patients was bigger than their free writing. Conclusion The present study is noteworthy, because it is the first attempt to use a software program to objectively quantify parkinsonian handwriting size. This measuring system would help clinicians monitor handwriting size throughout the disease course. From the perspective of rehabilitation, language‐appropriate external visual cues might be useful in daily communication (i.e. handwriting) activities in Korean patients with PD. Geriatr Gerontol Int 2014; ●●: ●●–●●.