i've never really thought about using anything bigger for my hihats than 14". most of the time i actually use 13".
for the last couple of days i've been talking to a drummer who is playing lots of hip-hop and he turned me on to trying out 17" hats.
i just returned from our little drumstudio where i tried a mb20 17" heavy crash as bottom, and two different 17" tops: one was a byzance jazz 17" extra thin crash. the other one a byzance traditional 17" thin crash. the first combination (with the extra thin crash) sounded real washy and deep. the second version with the byzance traditional 17" thin crash however is my new favourite. it has a deep and powerful stick sound. a little bit more articulate than the first version. unmistakably noticeable as a timekeeper. what i liked about it the most though was when i played it by foot. it has a real nice, warm, deep chick-sound that really completemented on the ride pattern i was playing. i guess i found a new hihat which i am going to use with my band tomorrow. lets see how it works there, but i am sure it will inspire my drumming and make me play slightly differently.
what i am trying to do with this really is to encourage you to look into your cymbal bags if you have any unused cymbals which you could use as a new hihat combination, stack, or any other efx-sound. it will defenitely inspire you in your own playing too. have fun!