168 is 5'6 so.. either you aren't 5'7 or the skis arent 168.. im guessing the former.. no head height is about right give or take a few cm.. depends what ski and what person, and how much you weigh..
It will be fine then. Personal preference. I was 5'9 140 and rode a 169 Fujative when I started park, it was great, no regrets, now I ride 178 Anthems and I'd be fine with a mid 180 park ski, but Anthems are fine.
well depends on your weight, size and ability and also what do you want the ski for... but yo would want to get something near 175 for park and a little bit bigger for BC imo