1) By pass capacitors are used to prevent any voltage spikes that could falsely turn on the transistor. Effect of capacitance value is that of a low pass filter - higher the capacitance, lower is the frequency of spike which the cap can bypass. Eg- if a 1uF cap by passes all frequencies greater than 1khz, a 10uF cap would pass all frequencies greater than 100hz also.
2) Effect of coupling capacitance value is that of a high pass filter - higher the capacitance, more are the low frequency signals which the capacitor will prevent. Eg- if a 1uF cap passes all frequencies greater than 1khz, a 10uF cap would pass all frequencies greater than 10khz only.
Karthik
