The method you use to change the resolution of your fluid depends on whether the fluid has square or non-square voxels. Fluids have square voxels when the X,Y, and Z Resolution values are proportional to the respective Size values. Otherwise, a fluids have non-square voxels. You can set your fluid to have square voxels by turning on Keep Voxels Square. See Change fluid resolution.
To change the resolution of a fluid with square voxels
For 3D fluids, the default resolution is 10 10 10, for 2D fluids, the default is 40 40 0.1.
Even if none of the fluid properties are defined as grids, increasing the resolution increases the quality of the rendered fluid.
To change the resolution of a fluid with non-square voxels
For 3D fluids, the default resolution is 10 10 10, for 2D fluids, the default is 40 40 0.1.
You can also change the resolution of a fluid from Edit Fluid Resolution Options window. See Fluid Effects > Edit Fluid Resolution.
Even if none of the fluid properties are defined as grids, increasing the resolution increases the quality of the rendered fluid. Turning on Keep Voxels Square can also increase the quality of the rendered fluid and reduce fluid render time. See Keep Voxels Square.