Regarding monitors of different size, I'd think that since we have access to the size of each monitors, and we have the monitor's "index" number, it's not difficult to calculate the maximum rectangle area coordinates. In fact, the computer will have better chance to accurately fit to the maximum area.
And the dot pitch is seldomly a problem for people who want this configuration, because they tend to buy the same model at once. For people who can't, or have to buy a different one because of broken monitor, they'll understand the visual experience would be suboptimal.