I personally don't think that 17s are "too big", but they are the MAX for me (and with some companies now making {much} lighter 17s to fit our rides, even the weight weenies cannot use their previous mandatory
heft as a rationale for them being "too big"
).
But yes, of course if they want/
need that super 'cush effect' of a very tall side wall, the only way to get it (and not have to worry about rubbing /odo way off, etc.) is with the smaller diameter wheels.
18s
would be way too big, unless you just
had to use them in order to fit some humongous sized BBK under them, but then you have all of the extreme negatives of a HYPER 'rubber band' effect side wall (basically, NO side wall!
).