in the past "most suitable" board length was associated with height. from that perspective 5'11 is a bit short for a 160, especially if you want to ride park features. more recently the association has migrated much more toward weight rather than height, and from that view, 190 gels well with a 160. These are just general guidelines though. as wiredsport said, different boards include different features that alter these. however, most people youll see on boards do not have so-called "mathematically optimal" set ups and do just fine. unless your board is grossly over- or under-sized youll still have loads of fun.