RF Design / Calculations - 433.92MHz, PCB antenna, values of capacitors
This paper from Epcos gives some hints on SAW transmitters: http://jap.hu/electronic/rf/434MHz_SAW-based_oscillators_and_transmitters.pdf
Basically, C2 and C3 provide positive feedback and also tune the antenna to the desired frequency, what improves efficiency. C3 (emitter capacitor) usually is bigger than C2, but the actual values will always depend on unknown parameters like stray capacitances (transistor, pcb) and the antenna loop inductance.