Get the next biggest size that will fit into the amp.
as For setting the 4 channel, a general easy to do tune would be to set all the settings on your head unit to how you like em, with the gain down on the amp, turn your head unit volume about half way, or to how you would normally listen to it, then turn up the gain until it sounds good ( highs, mids and lows) then you can mess with the other settings, it really depends on what sounds good for you
you could also use a multi meter, you would have to get the rms of the amp which is 75x4 watts rms so thats like 18 watts per speaker, then find out what your speakers rms is
turn all the settings on the head unit down, use a multi meter on the -+ on the amp speaker outputs, have the head unit up about half way, and start turning up the gain while watching the multimeter, and ocne the meter gets to what your speakers rms is stop there
