The optimal size for CTA buttons in email marketing for mobile devices is typically around 44x44 pixels as a minimum, per Apple's guidelines. However, a height of 50-60 pixels and a width of 200-300 pixels is often recommended for better visibility and engagement. This size ensures buttons are easily tappable and legible on smaller screens. Additionally, using a font size of at least 16 pixels helps the text stand out. Overall, the goal is to create buttons that are prominent yet not overwhelming, encouraging user interaction without compromising design aesthetics.