I'm 88 kg and 95kg in winter wetsuit and ride a fone rocket s 70 litre 5'2 x 24 board sailing coastal conditions and waves in UK west coast.

I'm trying to decide about a bigger board as on rare conditions the 70 is hard to get on eg onshore small period waves and light winds when it can be sub 10 knots on the inside.

So a bigger board to complement my 70 such as

1. a DW 120 d 7'5 x 20"

2. Or mid length eg 6'6 115 x 22" or 6'4 105 x x 21"

3. Or alternatively something like a 5'6 x 22.6" 83 litre as a single board

I fully appreciate the 120 gives me sup foil options that the other two probably don't

My question is - is there a law of diminishing returns in terms of board size. If I'm okay on a 70 95% of the time, is the DW overkill? I admit the idea of a seven foot board doesn't really appeal (unless sup).

Would the single 83 board provide any noticeable better get up and go than my current board?

What size mid length will give me enough float without being cumbersome?

What do you think?
Can't demo hence all the questions

I've gone from 4'7 x 70L to a mid length 5'10 x 85L which is about +10 for me, for similar reasons to what you mentioned.

love everything about it and not going back. It opens up conditions that are much lighter, I can use my small wing all the time and the get up is exponentially easier.

the length difference is more than offset by being more forward and the narrower board feels much more responsive.

same weight as you, and i've used older style F One 75/85/90L boards, 5'7"*22" Omen 72L and 7'2" Armstrong DW 106L. I'd pass on the the DW unless you really want to DW/SUP. I didn't, and ended up selling the DW Armie. Bottom line is it gave me maybe 1 knot lower than the Omen and i found it annoying on the water...and everywhere else. The Omen shape/size is a one board quiver for me and i don't need anything else.

I'm a big fan of the KT Dragonfly 7' 19" 100L at my 87kg. My first consideration on a board is how wide it is. I don't want anything over 20". Narrow boards surf just so much better. Another great thing about this board is the biggest wing I will use is a 3.5m again making surfing the waves so much better. Length will also smooth out the ride.
Board in action:
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Agreeing with others and adding a few things:

I think you'll learn the width is more cumbersome than the length. (like foilthegreats I wont ride over 20" again either)

The liters you're listing are overkill. A neutral to slightly positive volume midlength at 20" wide will check your lightwind box vs the board you currently use. I'd go 5'10" or 6'2" Carver if I were you.

The diminishing returns come into play with the DW board if you don't want to change your sail and foil sizes. If you plan to keep the same kit then you will likely be disappointed in a DW board. If you want the option to ride smaller foils and sails though, that's where the benefits of a 7'+ 110l'ish board will start to show.

I've got a 8'4 DW & absolutely love it. Somedays I'll be looking forward to a light wind session as with a big foil it is so cruisey. The only downside for me is the arse gets hit by chop but it carves & turns as well as any board I've ridden. I also have a 7' & 6'6 midlength. All similar Kalama style shapes & all 20" wide. The 7' only rocks side to side which you get used to after 20mins so is stable, gets up to speed very quickly & very easy to get going with hardly a pump. I don't really think there is much difference between the 6'6 & a normal toast shaped board. The 6'6 moves on both axis' like a short board. I use it for windier rougher days but that 6inches makes a big difference. All these boards however are light years better than the fanatic skywing I had before