Well it's not a real powerful mixer only pushing 150 watts per channel into 8 ohms. Most of the PA speakers out today require anywhere between 300-500 watts @ 8 ohms on average. Is this for home use? If so you could get away with smaller speakers. If it's for club use, you may be in trouble. Typical "karaoke" speakers will probably be the better match as they typically do not handle high power, however they are grossly overpriced for what you actually get. Two examples that mate perfectly to your mixer/amp
http://www.acekaraoke.com/hbmbsp450v.html
http://www.acekaraoke.com/acesonic-sp-4 ... ystem.html
I believe these are prices for single speakers on the second & pair for the first link. Not positive, but for that kind of money you can get really decent club level PA speakers. Again, for home use, these may work just fine.
Whatever you get you want the "PROGRAM" power rating of the speaker to match the power rating of the amp @ 8 ohms. There are SOME 4 ohm speakers out there, but they are harder to find. If you can find a 4 ohm speaker, then you want the 'program' rating of the speaker to be able to handle 300 watts @ 4 ohms.