Common Emitter Colpitts oscillator
C5 is relatively large, it decouples the emitter to keep the AC gain high. 

L2,C2,C3 form a parallel resonant circuit. L1 is simply an RF choke (high impedance at oscillator frequency).

C1 can be relatively small as it provides positive feedback to the transistor's base which is a relatively high impedance node - its actual value will depend on Cbe and (Ccb * voltage gain) - the latter being the Miller capacitance. These (Cbe and Miller capacitance) are effectively in parallel, and will attenuate the base signal.

C1 will also depend on the ratio of C2 to C3 which provide the ground tap on the resonant circuit: if C2 > C3 then X(C2) &lt; X(C3) and the tap is closer to L1, reducing the positive feedback voltage via C1.