I am not sure if you already have left and right speakers by your very limited post. If you do, why would you be looking at putting in another type of speaker that doesn't match the voicing of your stereo setup? Besides to seriously answer your question, placement within the 5.75" space becomes important. For example, is there some type of top and bottom molding that might interfer with the drivers? How far back from the plain of the entertainment unit will the speaker be placed? You might be better off with a speaker that doesn't fill up the space and has some breathing room. If you cram the biggest speaker you can in the space, it is probably going to sound really boxy.
Anyway, give more specifics and I, as well as others, can give you better advice.
Gregg
