Two Fluid bouyant problem and mesh
 

I'm trying to simulate a problem of two layered fluids of different densities, such as Rayleigh-Taylor instability or double diffusive
My system consist in two layered fluids of different densities. (At first, without using scalar quantities with different diffusion coefficient, but I include it in the future)

When I want to simulate the Rayleigh-Taylor instability, noticed that the result varies with the number of cells that use. I tried to refine, but the result is always different.
My system is simple, just a rectangle defined between (-1, 1) for x coordinate, and (-3, 3) for y.
The first fluid is defined between (-3, 0) and the second between (0, 3).

My question is, How I can I know the proper size of cells so that the result is correct? Maybe any problem with the boundary conditions?

For this problem I use a inviscid flows and boundary conditions are continuity system for the coordinates, and wall to the x coordinate

Please any help is welcome

THANKS