Amplitude of sine wave with minimum distorsion
Lets predict a gain, then decide the input voltage.

Ignoring the loading of the output emitter follower(which may dominate), the gain will be


R_earlyVoltage / reac


and with both those numbers decreasing as current increases, lets just divide


EarlyVoltage / ThermalVoltage 
26 volts (assumed) / 0.026 volts (fact)  


thus the approximate voltage gain will be 1,000x.

Since 100 uv * 1,000 == 0.1 volt, small compared to rails, 100uv seems good.

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Since Q3 is diode and Q4 is common_emitter operating, I'd expect substantial output DC error.

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best  ?      It's good enough. Since we have no metrics other than sin_ in to become sin_out, go ahead and explore.