Usual ride at Saunton, on a Gong SUP , the smallest being a 6'11 , works from knee high to head high, but definitely easier to balance on if it's glassy or at least not too much chop, also have an 8'6 fairly flat rockered board (Gong Empire) which is easy to ride, if you surf, you can easily ride this.
don't have a "longboard" SUP at the moment, but should be getting a Gong around 9'2 ish fairly soon, very similar to classic longboard shape.............. anyway
if you are down at Saunton, you are very welcome to try these boards, whether you have ridden an SUP or not. I'm the older, bald guy, using red boards. ..........[ no I don't have an financial gain, they give me some discount and I like the brand!]
here's a link to a "longboard" style video on their website, the rider is Patrice Guenole, owner/shaper of Gong.http://www.gongsup.com/Too-long.html?lang=fr
the rider is around 105 kilos, and rides boards as short as 6'8, Sups don't need to be like boats !!