Hi Peaveyt60 and Welcome to Guitar Noise! Glad to have you aboard.
First of all, I went to Peavey and found the manual for your XR600B Powered Mixer.
Click on Support, then Search Manuals. Then type in xr600B. They have your manual.
OK, your Powered Mixer has only one amplifier built in (most modern units have two). It provides 300 Watts with a 2 Ohm load, and 210 Watts with a 4 Ohm load. Ohms is resistance. The higher the ohms, the higher the resistance. So your amp can provide more power with a 2 Ohm load versus a 4 Ohm load.
Now this is not a real powerful amp. But for vocals and acoustic guitar it is plenty. You will be able to play small clubs easily. For a Rock band it would be pretty underpowered, but could be used if everybody kept their volumes down.
Most speakers are 8 Ohms resistance. There are some 4 Ohms, and even 16 Ohms speakers.
Whenever you run two 8 Ohms speakers in parallel (which the two speaker connections on your XR600B are) you get a 4 Ohm load. So with two 8 Ohms speakers attached your amp will produce 210 Watts. If they are identical speakers they will split the power and get 105 Watts each.
If you could manage to find 4 Ohms speakers, then two 4 Ohms speakers in parallel make for a 2 Ohm load. Then your amp would put out 300 Watts. Again, if the speakers are identical they will split the power and get 150 Watts each. This would provide the most volume you could get from your Powered Mixer.
I don't know what your budget is, but if you can afford it, go with 15" speakers. This will really bring out the fullness of your acoustic guitar. But you could save a little with 12" speakers and they will sound very good, but probably not as good as 15"s.
One of the very best and economical 15" speakers around is the Peavey PR15. These are probably the very best sounding speakers under $200 (each) around. They get great reviews. I know people who own them and love them. I have heard them and they do sound great.
Read the reviews. These are great speakers, especially for the price.
If this is outside your budget, go for used speakers. You can often pick them up for half or even less than new. Yamaha makes great speakers and can be found everywhere.
With two 8 Ohms speakers you can have one aimed at the audience on a speaker stand. You could turn the other around facing you for a monitor. This is a very common method and works well.
You could actually run four 8 Ohms speakers off your Powered Mixer. This is a 2 Ohm load. If the speakers are identical, each speaker would get 75 Watts each. With this setup you could have two Main or FOH (front of house) speakers and even two floor monitors. This would work well for a small acoustic group.
I hope this has helped. If you see any particular speakers you are interested in, then post back. Myself and others will be glad to give you our humble opinions.
But in all seriousness, many here have PAs and have a pretty good knowledge of different manufacturers and speakers.
If you know something better than Rock and Roll, I'd like to hear it - Jerry Lee Lewis