This just happened to me:
We want to buy a rights managed photo from a photo agency (PA):
Me: - We will use the photo on a website for a year
PA: - Okay, will it be below or above 3x3 cm?
Me: - Well Miss, on the web we work with pixels, thus the image may be under 3x3cm on large monitors :)
PA: - I see, but there must be a design phase where you calculate in cm, just like in print