The evaporation rate per unit area would depend on many factors, including temperature, how much water is in the atmosphere, pressure, probably what is in the water, and surely a host of other factors.
The evaporation rate from the pot as a whole at any given time will then be that rate per unit area, which depends on that host of factors, times the surface area.
So once you know how deep a layer of extra water you need to add to cover evaporation in one pot, that will be the same depth of water you need to add to a different size pot if you are cooking in that different sized pot at the same temperature, pressure, etc. as the first pot.
Think of it this way: Imagine you have a rectangular pot, with a removable divider in the middle that effectively turns it into two smaller pots side by side.
If you put the divider in and cook some rice in the left side, you need a certain depth of extra water. If instead you cooked that rice in the right side, you'd need the same depth of water there.
Now imagine cooking in both sides simultaneously. You'd still need the same depth in each side as when you were cooking in that side solo.
OK, now imagine cooking in both sides, but without the divider installed. That's not really any different than with the divider installed. Each side is still going to need the same depth of water as before.
(Well, there will be some difference. Not all heat gets into what you are cooking through the bottom of the pan. The sides get hot too, and some heat comes in that way. Remember that the evaporation rate at any given point on the surface depends on conditions at that point. A spot in the center will take longer to reach a given temperature than a spot near a side, and so we'll have a non-uniform evaporation rate across the surface. I don't think this will make much difference as far as rice cooking goes, but figuring out for sure would require knowing a lot more about how heat moves within a batch of cooking rice than I do).