In the magnetic ballast type fluorescent lamps (old ones),what is the need of a capacitor in the starter circuit and what determines it's ratings? If my understanding is correct its a bi-mettalic strip opening and closing producing an inductive kick,so it should work fine without the capacitor too.
The capacitor in old Fluorescent Starters is for EMI suppression. This is typically a fairly-small value - anywhere between 1n to 100n, depending upon who made your particular starter.
The capacitor may also reduce contact erosion on the starter contacts - I honestly don't know. But I do know that in olden days when everyone had an AM radio sitting on the kitchen counter, you could immediately tell if someone turned on a Fluorescent lamp that didn't have that capacitor inside the starter.
The capacitor is (in most common fluorescent lamp circuits) is for power factor correction. Since there is a coil in the ballast, the capacitor is used to bring the power factor back towards unity. Probably not such a big deal when you consider individual lamps in homes, but when you start looking at hundreds or thousands (aggregate of homes or a typical business), keeping a unity power factor is important. For business power, at least in my area, there is a steep penalty on power costs if the power factor is not close to unity.