Optimum size of the images I can insert in PowerPoint?

What is the optimum size I can do insert in Microsoft PowerPoint so that all the TVs and different projectors and laptops with different screens can reliably present good looking slides and nice experience for the end user. I cannot simply increase the quality of the image alone. I have to fix on the optimal option for the pixels, vertical and horizontal, of the image when used to display on full screen. I display only images and no text or tables will be there in slides. Thank you
