i'd recommend going through the technical documentation and the training example on mesh controls
there is no way to directly control the cell size, just the basic size and the number of times it is refined
what I don't understand is that you say the solution matches the navier stokes solution but there is still an error, do you mean relative to physical? if so, changing the cell size probably won't help because flow is solving the same equation
Start with the basic mesh, and turn off the automatic settings. This will let you control exactly how many cells there are in a given orthogonal direction of your domain. Since you control the size of your domain, this is essentially giving you control over the basic cell size. There's even a handy preview of your basic mesh from this control window.