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.
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.