Again, adding to what RAM said, it still depends on more detailed factors, which is why I offered to do a design for you. It seems as though you have a general idea of what you want, but your explanation is general as well. Here are a few Qs for you. Do you want more numerical loud, or intensity type loud or a good mix ? Are you shooting for a specific frequency range? Is the 35Hz still desired as a tuning or lower or higher? This is needed as the compression of the design will help the port in determining that desired tuning.
Now, though efficiency may increase with a larger design, this is true only for the lower frequency range around your tuning unless proper coupling of the port is controlled more for a broader range of efficiency for even higher frequencies. The design will likely be sensitive to higher power with higher frequencies above the tuning more so than below, so it may true that a larger design, if possible, be implimented to keep it from killing the highs and dropping fast below cutoff. But this also raises the concern of passband width as for most designs to be more efficient, they have to be optimized for the vehicles desired frequency response, and unless it is a fairly large vehicle, the tunung low cutoff may reside around the low to high 40hz range for quarter wave cutoff, and around the 90Hz range for actual high end, low impedance true efficiency when loading is attrubuted. So, tuning lower may not be good in that sense, where actually a smaller design may couple it better to the vehicle. But that would depend on what the vehicle will tell you it likes for a response. For example, my compact SUV enjoys a perfect response from 31Hz-125Hz as the trunk space is part of the cabin compression area, so lower tuning optimized for around halfwave of 62Hz will do some damage to an spl meter. And if utilizing quarterwave cutoff, will get remarkably low down to around 17Hz with the 31Hz cutoff. So, its all about details in what you desire. This, to me, needs to be known before applying any kind of box volume gueasstimate of actual desired loudness, as it may be different perspective between spl metering and just plain loud. There is a tradeoff, and that tradeoff needs to be known before any design is implimented.
