How to figure out the value of capacitors needed? 

Use the formula below to approximate the value of capacitors needed:

CL=((C1 x C2) / (C1 + C2)) + C stray

C stray is the stray capacitance in the circuit, typically 2-5pF.

If the oscillation frequency is high, the capacitor values should be increased to lower the frequency. If the frequency is low, the capacitor values should be decreased, thus raising the oscillation frequency.

If CL =20pF, C1 and C2 will be approximately 27-33pF each, depending on the amount of stray capacitance.