I think the term short board is all relative. It's more a style of surfing than a measure of length IMO. I have heard people say that they can shortboard on their longboard and things like that. I grew up on a thruster and twin fins but switched over for a few years to a longboard before dropping back down and I know I was guilty of shortboarding my longboard, lol.
In 1967 Nat Young's 9'4" was considered a shortboard. It's all relative. For me it's based on board shape and your size. Some big guys ride a 6'8" shortboard, but for me that is approaching either semi gun or funboard. I ride a 5'6" fish, but not everyone would call a fish a shortboard.
Yeah I ride a 5'6" as well and I'm 6'1" tall if that's not short I don't know what is. When I started surfing there were the older guys on longboards and the young guys were riding stubbies and twin fins. At the time they were considered shortboards. After the thruster came out everbody started riding really thin narrow boards and then that became the "shortboard". Back then the boards were really short, to short IMO. I remember going into a surf shop in the early eighties and finally deciding to say screw it and by an appropriate sized board a 6'4" shortboard. The guy at the shop said I think your going too big but if that's what you want go for it. Turned out to be the best board I had ever ridden up to that point. I guess a fish is now considered an alternative shape no matter how short it is. This all goes back to my original assumption that it is more about the riding style than the actuall board.
hell yes, your 6'4" is a shortboard. i'm not a small guy...my good wave (shoulder-2ft OH) shortboard is a 6'4". who gives a crap what others think? if that's what you're comfortable on, ride it! not everyone who surfs is a pint-sized lightweight...
But... setting the history lesson aside... in today's vernacular "shortboard" refers to the design and the intended approach to wave riding more than the length. "High Performance Short Board" (HPSB) is generally a highly rockered, thinly foiled board designed for modern surfing in waves that don't require specialized equipment. They're usually thrusters or quads and have some kind of concave bottom. Some people will argue that you can have a twin fin or single fin shortboard, or that a shortboard can have a flat or flat-to-vee bottom, and that's true, but the generic "shortboard" is neither - twins are more often referred to as twins, and singles are generally referred to as retro-whatevers.
So I'd say the maximum length for a shortboard would be between 6'8 and 6'10, depending on the size of the rider. Once you hit 7'0 you're really talking about some kind of stepup or semigun type design, or a hybrid or funboard. Because at that length, design paths start to diverge and you get boards designed either fall into the big surf category, or the "easy to ride" category.
I have enjoyed all of the input on this and have always pretty much agreed with the notion that less than 7' is a shortboard, 7' - less than 9' is a hybrid or funboard or mini-longboard and 9' or more is a longboard. But with all of the various shapes (fish, biscuits, etc.) and sizes out there today, I like to go with the notion (earlier expressed) of how a board is set up to ride - either as a longboard or a shortboard. It is interesting to note that the Eastern Surfing Association defines a "shortboard" for the sake of competition as any board that is no more than 2' longer than the competitor's height. At 5'7", were I surfing in competition (legends division ), I would haul out my 7'6" Jesse Fernandez WRV funfish in small surf and catch a whole lot more waves than on my 6'6" thruster!