Bigger and smaller boxes have trade-offs.
The smaller the box, the peakier the response in the upper bass/lower midbass region (which is what most people interpret as "tight" or "punchy"), higher mechanical power handling (less efficiency), worse low frequency extension (how low it plays), degraded transient response (sound quality), and overall more output. These are the effects of a "high Qtc" box.
The bigger the box, the flatter the frequency response, the less overall output, better transient response, better low frequency extension, and lower mechanical power handling. These are characteristics of a "low Qtc" box.
Now, "how far off" from recommended you can go really depends on where on the "Qtc" scale their recommended enclosure is, and how much box changes affect the "Qtc" of the system. It is not uncommon for manufacturers to recommend boxes on the smaller side ("higher Qtc" boxes) because it gives off the type of bass that the typical "bass head, street-beater" will like. Doesn't necessarily mean it's the "best" box, because what makes the sub sound "best" is all personal preference. Some people like the sound of low Qtc (bigger sized) boxes, some people like the sound of high Qtc (smaller sized) sealed enclosures.
So.....after rambling for a little bit and probably losing you along the way (I'm really bad at explaining stuff), the point I'm getting at is that there isn't any set amount of variation you can/can't have, and being "over" or "under" the recommended box size is really personal preference and dependent upon what type of characteristics their recommended box size has.
So, it really depends on what sub in particular we are talking about, aswell as your listening preferences and power requirements. However, I will say that slight variations in box size for sealed enclosures typically have less of an impact on performance than variations in box size for ported enclosures. So if you measure wrong, or don't have quite enough room or whatever....it isn't the end of the world and the sub won't self-implode because of it.
