If an amp give us some specifics as in brand and model number if you have it.
Coaxial speakers will generally be fine with 14awg or 16awg cable and if we're talking about subwoofer(s) then I generally suggest 10awg or 12awg.
These sizes are overkill to some extent but speaker cable is cheap and I'm a firm believer in the adage that "It's better to have and not need rather than need and not have."
Amplifiers are a whole different animal altogether. Some amps can get away with as small as 12awg or 14awg (dependant on output levels) and some require as large as 1/0awg or larger.