a bigger box will give you lower frequency extension..... to a point. past that point the box will not offer enough suspension, depending on the driver.... at high power with a box that's to large you will increase the chance of bottoming out the sub or loosing control, which can sound like crap & damage the sub
I like my boxes on the large side of the scale for sound quality, but especially if your running them at or above max rms, try to stay within limits.... a 1/4' of airspace too much can make the difference between an awsome solid boooooooom & a fluttering fart....
only use dampening in the box if it's construction is a little less than solid, if you've got a big flat span it won't hurt there, but is unecissary for the most part if the box is constructed properly, your better of bracing anything you feel is going to resonate than dampening it, because dampening will do nothing for flexing
if the box is to big you can always fill it with extra bracing.... complicated shapes are best built on the large size because it is difficult to acurately calculate total dimmensions before build, it is always easier to take some airspace away than it is to add airspace... the only way to add is to overstuff with polly foam, which can make the box appear up to 10% larger to the sub, taking away airspace is always easier
the best analogy I can give of box size is picture a mountainbike tire riding down steps... the steps are the bass hits..... very high air pressure in the tire will give a stiff hard quick shot....it is safe though in as far as damage is concerned..... this is the equivelent of a small box... punchy bass with less low extention
now picture the tires dropped to like 25-30psi...... now she rides down the steps alot less punchy... smoother if you will.... this is the equivlent of a large box.... smoother bass, less punchy & even low bass extention... problem is if the pressure is to low(box to large) then you may smash the rim into the stairs on a big hit(bottom the sub if you didn't get that yet)
to be safe run the box a little small... for best results run the box larger, but you should mesure the box after it's built & if it is larger than manufactures specs call for, reduce it till it's within tolerances, if you have a good ear you will be able to tell if the box is to large by the subs response....
are you doing ported or sealed?