There is no maximum wattage, but the minimum wattage is the wattage per channel. If the stereo can output up to 50 rms watts per channel the speakers should be chosen that can handle that much without distortion. Also, match the impedance, which is usually 4 ohms. If you get more powerful speakers, they may last longer, but they won't make the stereo more powerful. You also need to strongly consider frequency response Of the speakers. They should be able to reproduce efficiently, the whole range the stereo gives them. Treble is easy and requires less power than bass. So if the low end of the response is 40Hz or lower, expect a stronger sounding performance than with a speaker whose low end is 100Hz and that will sound tinny. That stereo actually outputs only 14 watts rms to 4 channels and about 22 rms watts to the sub output channel ( 2 ohm sub).