Speakers are fun..... once you have some understanding of how things work.
This link will take you somewhere that has a pretty good tutorial on resistance and power.... http://www.prestonelectronics.com/audio/Impedance.htm
Either way your extension speaker should be 8 ohm (4 ohm total load in parallel with the 8 ohm speaker in the amp) and I would likely size it at the 60Watt RMS which is the output of the amplifier (or a bit higher for headroom). Bear in mind that the 100 Watts referenced in your spec is now split between 2 loads (or 50 Watts per speaker).
Watts is Watts, Amps is Amps, to my mind the more important thing is the frequency response and range of the speaker.... that is what really matters in music, the part you hear. Volume is a function of size, not power, and you will try to get something loud without losing the high frequencies...... do your homework and check those numbers as well. You should be able to find something in your price range that will serve you well without "letting the smoke out" of your amp.
Take Care, and Good Hunting!
Doug