You should try what works, and get experience. About max coil depth clearance, basically, xmax one way+15% and a little more for a ported box sub is good. For a sealed box you may want a bit more then 15% extra.
The variation of winding height(xmax), and coil density change the characteristics of a subwoofer alot! Really, you need experience and an undertanding of subwoofer design. There are alot of newbs who blindly throw parts together and throw it in an average box, and some who add too much spiders with too little power applied to the subwoofer without understanding what character they will end up with.
I say to model alot of different subwoofers first, and read about subwoofer physics, after that you will know how a longer coill WH changes the tsp character of the sub etc...
You can take anyone advice on how long of a coil you can fit, but you may not be told that too long a coil has too high inductance, weight, and may make your Qts too high and make your subwoofer a sealed box sub instead of a ported one.
You also want to be carefull about the gap tolorance which is the loosness of the coil in the gap, too little will reduce bl/raise Qts, too tight and the subwoofer will have nasty power compression, for instance, if it is too tight you may only be able to max the output of the sub on 800w when the coil should be able to take more and if it does the output will just be limited by the ceiling at 800w.
Sealed box subs need good xmax for decent power handling with decent sound and output all together, ported box subs are alot more efficient with less xmax and you need to design the box corectly to get it all to work good.
For clearance you can start with a deep motor clearance and average size coil.