At about 103kg I can paddle my 120ltr 7'4x30 board ok but any bump on the water or currents moving the water around and it's too much work. Because I'm interested in as much maneuverability as possible, I'm wondering if it could it be easier with the board actually under the surface of the water a little, a lower volume board?

Another question. Would length add stability to the same volume and width board? For example, an 8'2 x 30 with 120 ltrs

Colas is probably best to reply with low volume for 100kg's...

IMO, yes length adds stability, as does a wide nose / parallel outline. A board under water to paddle is fine, however it is way less efficient to paddle and stand on, so it is way more tiring. Sessions that used to wear you out in 2hrs can wreck you in 30-40 mins. Whether the trade off is worth it to you, that is the relevant question. I've also had longer boards with thinner ( in thickness) noses that felt just as maneuverable as a shorter board, but the added length gave much more stability and ease to paddle into waves.

+1 with Hoppo.

As for lower volume boards:

pros:
- it is less moved around by chop
- you get less the "tree trunk" or "cork" effect where trying to correct a roll on the side, the board will roll happily on the other side, as if on a longitudinal axis

cons:
- the sweet spot to balance on becomes quite small, especially fore/aft
- correcting a roll makes the board sink, acting like a "shock absorber". It is easier to tame it... but it is more tiring: it is like piloting a submarine rather than a floating log
- the board becomes a "sea anchor", a drag to paddle.

My advice, that I implemented in my new custom boards:
- get enough volume to paddle with some efficiency. For my 97kg it meant 112 liters, a guild factor (volume/weight ratio) of 1.15. I think 1.15 to 1.20 is a good compromise. I can use a 105 liters board for 45mn before making too many mistakes from being tired, but I can have 2h sessions with a 112 liter board and 3h with a 120 liters.
- keep some width: better have thinner rails because you can spread the volume into a bigger board surface than a narrow board with meatier rails. Domed decks or step rails will help, too.
- keep some meat in the rails in the mid section. Thick rails center and thin rails on nose and tail are nice because the board becomes quite nimble when digging the rail in turn: the mid rail acts as a pivot point to be able to change the turn radius at will where thin rails everywhere will tend to lock the board into its natural turn radius. Only pros will benefit from the ultra low latency of very narrow boards.
- add some length: with a thin and light nose, you wont add swing weight in turns, but paddling will be much more efficient, especially with some chop

I'm 6'2" and 100kgs give or take, depending on the season
My first board, just on 10 years ago, was a mammoth at well over 200L.
I slowly made my way down, hovering around the 140-150L mark, and now very comfortable around 125L.
I find that having the deck just above water, i.e. rails and tail under, is the sweet spot for me. I have tried boards that are completely under, with the water up to my ankles, but find it too bloody hard to paddle. Becomes too much like hard work, not fun.
I recently acquired a cheaper "fat boy" board for when people come to visit, and it is well over 200L. I found it really difficult to stand on, very corky.
For me a longboard shape with more width in the nose is so much more comfortable waiting for waves. Leisurely even. I'm not that good a surfer to need a pointy nose, even though I do have a couple on the rack, and have enough weight to throw a longer mal shape around effectively. The longer and more narrow you go the more "roll" will affect you.
So looking at the figures that Colas has mentioned, I'm sitting at a guild factor of 1.25.
The other thing that I've always had to consider is my height. I have a higher centre of gravity which also affects stability. I would sometimes love to be 4ft tall on a low volume ripper, but I am what I am (although shrinking with age)

Agree with previous comments. Specific to your situation, I would be going longer first adding surface area at least 8', I also wouldn't be afraid to go a little more volume if the shape dictates it, consider talking to a good custom shaper.

Thanks for the reply's. My board, with 120ltrs floats me no problem at 103kg. It's 7'4x30", pretty wide nose and tail. I'm a pretty efficient paddler on a short board. It's just very corky. I have the same board with 140ltrs and 32" wide and it's very easy to stand on even in rougher conditions. But I can feel the extra width.

I'm very interested to see what you find if you drop below ~1.2 L/kg -- I'm in about the same zone on a pulled in shape, but I can't see myself going lower. For dealing with chop, perhaps you've tried this already, but sometimes I sort of lean in to crap conditions by riding my smallest board. Yesterday started on my medium stability board cause it was choppy, then it got properly terrible out so I switched to my small board so I could work on balance.

That's my question. Could going down in volume actually help me? Maybe give me the same stability that I get with my bigger board by getting the board under the surface of the choppy water? At 120ltrs it's a little under the 1.2 ratio.

I'm also pretty tall so maybe just working on my technique is what I need. Maybe getting down a little lower in my paddle position.

A lower volume in the same shape should help the board roll less quickly side to side, but you will lose front to back stability smaller sweet spot as mentioned and it makes it harder to recover since the board continues to sink to one side easier once you get too far over. I think most would say adding length/width (surface area) is the biggest single factor in standing stability ignoring all other riding aspects. I think the gain you get in submerging the rails is nice but don't overestimate how much it will help you balance on low volume especially if you are already at 1.2 the rails should be pretty far in even with such a short board unless the shape is really strange.

Technique helps, I thought the supboarder pro video on paddling a small board was a good one, biggest help to me is to keep paddle in water and pulling or bracing as much as possible. If you are taller/heavier you have more leverage to turn a bigger board and cause falls when standing. Also consider how often you paddle, a really small shape you probably want to be going out 5 times a week to be tuned in to it usually only an option for pros and or young.

Not sure about that. 1.2 isnt that hard to padle. I,m 188x 95kg on a good day, with a lock on the cookie jar and after prope Sh#t (so mostly 98kg)- so 100kg with a wetsuit that we always need here and 49y old. For being a surfer I,m born at the wrong place of the earth as well (in the Netherlands) so proper waves are rare., making Supping mostly an holiday thing. My main boards are a Smik Hipster8.3x31x123l and the Spitfire8.6x30x125l when its big. Mind that i,m doing that with 1 foot that is very compromised having almost no feel and only a big toe propery functioning. Till 3y ago my foot was still slightly better/less worse, I was completlely happy on my Starboard Pro8.5x29x112l. a gunny pointy board.

Yeah, I think I'll work on my technique a little in flat water.