There are many ways to approach this. For example, I am looking at Microsoft Windows 11 hardware requirements. I see that Microsoft recommends a 720p display, which is commonly 1280x720. If your images are bigger than this, they will be downsized on this type of display, meaning that having something bigger (like 3000x2000) won't help. But this example is just for the minimum requirements for Windows 11. How about Macs?
I think the general rule is to go with as big a file as you can, until performance suffers. Big files take more bandwidth, however.
There are so many variables that all you can do is experiment and come up with something that you think is working.
Having said this, I create my videos at 1440x960 pixels, and I try to keep this aspect ratio, 1.5, constant. If I know that an image is going to take only have of my video's screen, I can go with half of 1440x960.
If I am off-target from what you are asking, keep asking. Perhaps be more specific.