you have opened a can of worms bigger than you could have imagined, time to get googling.
to port or seal, as well as the volume depends on the parameters of your sub, i designed a sealed box for my 10 inch sub a while back and it took a lot of effort.
but here are some basics.
if you want faithful reproduction of sound and have heaps of power to spare, go sealed
if you want fooley sick bass doof doof then go ported
a sealed box will generally be smaller than a ported one