what is the pixel size for web design in iphone?

I am designing an input form for a website to display it on iPhone devices. Now, I have problem since i don't know what is the pixel size I should use for. As i googled for some information to choose the pixel size for iPhone, i found many types for pixel size that correspond to each iPhone device from iPhone 5(640 x 1136) to iPhone XS MAX(1242 x 2688).

I don't know what size I should use to correspond to all iPhone device. Can anyone kindly help answer my question? Thank you.