Which blade size do you use for surfing?
Since my 57yo shoulders don't like my old Naish paddle with its stiff shaft anymore, I want to buy a new one with more flex.
Most brands recommend 85 - 90 sqinch or size L blades for my weight of 94 kg 205 lbs.
But the local shop does recommend size M or even S for higher cadence paddling.
What do you think?

I'd use a 85 at that weight. I've got a couple of KeNalu Ho'oloa's... with the flexiest shafts offered. The brand doesn't exist anymore afaik, but plenty of other brands do similar.

Naish has a wave paddle with a carbon/kevlar shaft, the blade size is not that big and te shaft has very nice flex

The Naish has 85 sqinch blade which is in the size L category of most brands.

I've used the same paddle since I was 107 kg, now 81 kg because I love the narrow blade WIDTH
It's 6.5" wide, 81 sq in
So much faster into a wave because my stroke is quick and effective right off the gun.
Easy on the shoulders.

When I use my 85 sq in paddle, that is at least an inch wider, it feels big & slow

That's interesting.
I'm looking at the KT/Quatro Uniflex Pro which is available in L (sold out) with an 88sqin blade and a 29mm shaft and a size M (in stock) with an 80 sqin blade an 26,5mm shaft.
The M is attractive but I struggle if it's too small for my weight. But with Rick's comment I may pull the trigger.