High resolution pictures

[omoeko]

Hi,

I would like to output photos for real estate interior, and would like to understand a few things. What sort of resolution should the pictures be finished at ? On one hand, I want high resolution photos, most of the photos will be viewed on apps or websites, I don't want it too big in size on the other hand if the pictures are viewed on a much larger screen, I don't want it looking grainy etc.

Can someone please help me on what high resolution picture output settings should be, and if the pictures are to be reduced in size for website optimisation etc, what should be done ?
To reduce photos, I usually send it via whatsapp and that compresses the size, but I want to believe there is a far better way.

Thanks in advance.