Ideally you're going to want the fluid particles to be as small as possible, depending on the type of fluid you want to simulate. For free-flowing liquids, i.e. water, milk, alcohol, you should have them under .5 cm. Maybe as low as .2cm.
.5cm - 1.5 cm: paint, liquid caramel or chocolate, motor oil, etc
Where you are now, 3cm-5cm is better suited for "clumpy" or "chunky" liquids: grape jelly, lava, vomit, raw sewage, etc.
Of course, the scale of your simuation needs to be taken into account, too. If you're simulating water being poured into a glass, then .2cm particles are good. If you're trying to simulate a 100 ft section of a creek flowing, then you should definitely make them bigger.
It's relative. Think of them as 3d pixels. The more of them you have, the better it's going to look.