Or if no one around you can always bail and flail, haha.
Oh man, never, ever, ever throw away the thing that will float you to the surface… 
Sorry, attempt at humor.
I just want to provide some reference. I hope others can too.
I am 78kg. I found a hypto krypto 6’0 at 37litres to definitely be too big.
A copy at 5’4, maybe 29litres at was just about big enough for someone with skill.
Oversize small wave boards could be more common than we think. Some people size up a bit and a bit is probably OK but it’s probably overdone too much
Edit: let me provide a bit more info. A full 6" of length with the extra width and volume that comes with that didn’t really seem to offer a lot more help paddling into waves. Maybe a bit easier but not much.
So I think yes, there is a right size for a person’s weight and this is topic is smart to bring this up as a subject.
I got a board too big before and then I complain when it doesn’t feel as good as I was hoping
Huck wrote:
get way up on nose and push down and hang on, that’s as close as you’re gonna get to a duck dive
There was a member on the old Surfline forum who said he could duck dive longboards. I believe his screen name was punkrock.
I asked how.
He said move forward some and push down on the nose. Shovel/scoop the nose side-to-side, making it go deeper. Get as deep as you can. Then, slide your hands backwards along the rails and push the nose forward. Grip tightly and hang on until you come out the other side. (I may have modified his method, but only slightly.)
I tried it with a 9-1 on a trip to HI. Worked well enough for the first wave of a clean-up set. The next wave of that set found me too close to the impact zone and sucked me back over the falls. Took some practice, and finding the right place towards the nose. But it worked much better than I thought it would.
Accept. Reject. It does not affect me.