How to calculate appropriate value for noise-removal capacitor?
A virtual ground is like a voltage source with a certain impedance. The size of the capacitor will depend on this impedance and the load you have on your virtual ground.

The noisy waveform looks like you have some switching activity that loads your virtual ground. For this reason it is better to consider the problem in the time domain. You need to figure out how large the current spikes are that are generated on your virtual ground and how much charge is transferred. Assuming a sharp spike all the charge has to be supplied by the capacitor and you will observe a voltage change of delta V = delta Q / C. The capacitor should be sized accordingly to limit this voltage drop/increase to fit your requirements. You should also be careful to place the capacitor close to the load in order to keep inductances low.