My 17.5", from this:
To, now you see it:
Now you don't:
What used to occupy the entire length of the passenger side of my Mazda Tribute, from glove box to rear gate, now sits in half the boot space. Contrast drop? Maybe. Would I prefer a tiny increase in contrast to a much more user friendly scope? Not a chance. Collimation more touchy? Nope, no more than any other Newtonian that is regularly transported. Designed properly, a collapsible dob is a very fine instrument.