#1 The volume of air 'trapped' in the box (internal volume)
#2 The area of the 'cork'/tunnel
#3 The depth of the 'cork'/tunnel

At any given box-volume a longer tunnel will lower the resonance (cos the weight of the 'cork' increases). A smaller area of the tunnel will do the same - think of cushion: if you press on it with the palm of your hand, you hand will only sink that deep - if you use only your fingertip using the same force, your finger will sink deeper; the cushion is seemingly softer now. Same thing with the smaller tunnel area - the air 'spring' in the box is softer to the smaller area, thus the Helmholtz resonance lowers.

And, of course, the reverse is true - if you make the tunnel shorter, the resonance gets higher, & the same thing happens if you increase the area of said tunnel.

The purpose of using this in a speaker box are several: Getting a deeper, more powerfull bass - helping the driver-unit where it has to work the hardest, in the bass area. It will typically reduce the movement (=excursion) of the diaphragm to 1/4 at the resonance - & reduce distortion from said driver with the help of the aforementioned reduction of movement.

One can, however, not just put any driver in any box using this principle - several parameters have to be considered; The drivers properties (socalled Thiele/Small parameters - a science in itself! :-) ), the box volume & where to place the resonance - it's here the tunnel/port size comes into play!

Sorry, that sorta got outa hand - hope it helped.