There is no specific set size. It is more what is best for your application. For a vented enclosure though I typically go with about 1.75 cubic foot tuned around 57hz. For sealed enclosures I typically go about .75-1 cubic foot.
This looks pretty good and should blend pretty well to the subwoofers. If you want to get lower than vented is the best option. I like to tune lower than the crossover frequency by about 1/2 octave to keep the rise in group delay out of the intended bandwidth were it will be used.