Tap-typing with a pen on a virtual keyboard is often used for intermittent text entry on a pen-equipped tablet; however, this has usability issues owing to the large size of the virtual keyboard. We speculated that tap-typing with a pen on a tablet may not require such a large keyboard, but there was no empirical study to help us to determine the best virtual keyboard size for it. Therefore, we conducted two experiments. In the first experiment, we compared virtual keyboards with different key sizes (3, 4, 5, 6, 8, 10, and 13 mm) using a text entry task. The results showed that keyboards with 6-, 8-, and 10-mm keys were the best when text entry speed, task load, and preference were considered. In the second experiment, a keyboard with 6-mm keys was compared with other conventional pen text entry options using a presentation slide editing task with a pen. The results showed that the keyboard with 6-mm keys performed better than the others in terms of preference, System Usability Scale (SUS) scores, and ease-of-learning scores in a Usefulness, Satisfaction, and Ease of use (USE) questionnaire.