The Onkio single disc does have a volume control knob. I had never used it according to the booklet. It is supposed to have a small power supply to use head phones independent of a receive or amp.
Pretty much any amp will work for the CD player. The speakers however depend on what your amps wattage output can handle. Example: an amp that puts out 89 watts continuous rms would be safe enough to drive 100 watt continuous rms speakers. Just don't crank them.
This unit is a single disc player. I just measured my player... 11 1/2 " deep, 17 + " wide and 4" high (including feet). Remember when considering the depth leave room for plugs and cables!