1K - 10K should be fine.
With equal-value resistors, you'll have 50% voltage divider on each side for a loss of -6dB (when there's just one signal).
You want the resistor values to be high relative to the source, and low relative to the amplifier input. So, it's a compromise and there could be more than 6dB of loss but you'll probably still have plenty of signal.
If the amplifier doesn't have a capacitor at the input, it would be a good idea to stick a capacitor (1uF) in series with the input (after the "mixer") to block any DC.
I don't need a pot since I can control the volume of the signals before hand.
Actually... A pot with the center-tap going to the amplifier would be another way of doing it (again with the optional series capacitor). And, that would allow you to adjust the "balance" between the two signals.