i think 28 to 29 inches for 14 ft boards. as dj said shape and rails play a big part. personally i like edges from behind the middle to tail and i'm keener on a more planning nose then displacement. displacement tends to part the water and drop the nose down. something i don't want because a sup just isn't going to go through bumps like a ski or oci. i want the nose just sitting in the trough up the front. its either there to drop over the bump in front or its not imo

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i also think there is not point in having a bit of rocker. you either have a rockered board or the flater designs in vogue at the moment.
again personally i'm having a great time of it on my rocker planning nose, back 'edged' board- made to dw ---after you are really surfing on a decent dw er and this design concepts give me the control of a surfboard



the tail rocker is important imo because when you stand back on a none rocker tail board the tail drags where a rocker tail still 'fits'. i'm finding it pays to hold back on the crest of the swell/bump and let the trough develop in front. i'm not noticing any drop in pace and your rested to really charge a section when its there to be had.
front nose rocker means you don't have to dart down the back or the board to 'save ' it all the time.
mind you one of the great thrills in sup dw ing is to be able to go back and be able to stay there on the good days
the longer boards you can generally drop down in width
cheers
disc just my thoughts