Hi A Vern,
I have a 5"6 and regularly ride it in head high waves, its a lot of fun. If the waves are pitching i.e. A low tide beachie or point, for me its a little tricky. However, having seen the video which you posted ,dont worry thats sweet p country. They re great when you have a fat wall to lacerate. I use contoller fins from futures and reckon that they are great. However, i d love to try the new rasta fins. Whether you prefer a 5"6 or 5"8 is a personal thing and depends on your prefered volume....if you re comfortable with the volume, the length will not be an issue.