There is no answer for the total area of the slot. However we can calculate the width of the slot, and as you say this is done with the same method used in How to find out the maximum radius of a hole that can keep water stay in a container by water viscosity?.
If we have a cylindrical meniscus then the pressure difference it produces is:
$$ \Delta P = \frac{\gamma}{r} $$
where $r$ is the radius of the cylinder:
So for a given pressure the maximum radius before the water starts flowing out it:
$$ r = \frac{\gamma}{\Delta P} $$
And the slot width is just $2r$.
The reason there is no maximum area is that this equation does not contain the length of the slot (actually it assumes the slot has an infinite length as it ignores end effects). So you can make the area arbitrarily large by making the slot arbitrarily long.
