Does anyone follow these? What would happen if the box is too small than what they recommend?

 

If your box is too small, you will have to fool the speaker into thinking the enclosure it's in is bigger...you can do this by stuffing acoustical foam, or fiberfill stuffing tightly into the box, and up to the top around where the speaker is mounted. You also have to be reasonable here too...don't try to put a 15" speaker in a 0.5 cuft box with 5 tons of stuffing. Your box should be as close as possible to the manufacturer's recommendations.