My answer would be, no. I guess it would also depend on what ones definition of seaworthy is. If the definition includes the ability to cover a lot more water efficiently, then length might be a factor. If we're talking about the ability for the boat to remain upright and handle rough water, I don't think length is the primary factor at all.
My personal preference for a seaworthy craft is longer than 15 feet, but that's just me.