Wound size is an important parameter in the evaluation of healing status of chronic wounds. Many technologies, such as software embedded digital camera or artificial intelligence assisted smart phone applications, have been applied to the automatic wound size measurement. However, these methods or devices are either expensive or inconvenient. Instead of using a ruler to measure wound size, we propose a novel method using fingernails as the reference objects with the combination of two deep learning models. The width of the nail was first detected and computed by RCNN deep learning (DL) model. After that, the width and height of the wound were inferred by those of the bounding box generated from YoloV5 DL model. The wound size can be obtained from the known nail width. The experimental results showed that the mean Pearson correlation coefficient reached 0.914 in comparing the prediction and the standard wound sizes. We believe our proposed model is a simple and effective method for wound size measurement.