PA system speakers are
typically 8, 10, 12, or 15 inches. Generally the larger the speaker,
the lower the frequencies they can handle.
For Medium Bands (typically one or two
guitars, bass and drums, vocalist and maybe a keyboard) we'd
normally recommend 12" speakers, but if your style of music is on
the heavy side and you are planning to mic up the bass through the
PA, then you should consider 15" speakers.