Possible Numptie question, Am just about to spec up a pair of handbuilt wheels, and am still unsure on the best rim width. Usually I have migrated my current xc wheels to my trail bike, and normally I think they are a 19mm rim. I have the choice now to spec up the optimum rim to the bike and type of riding I do.
I have a 140mm trail bike that I tend to run reasonably full volume trail tyres on i.e Maxxis 2.35 or specialized 2.3 which come up quite big. Doubtful I would ever go wider than that tyre wise, so is a wider rim a better option or overkill (was looking at Mavic 521) or should I just stick to 19mm like I have used before (dtswiss 4.2D)

I love wide rims. Less roll off in corners. Wider is better right up to the point they get too heavy. Other than weight, I don’t see a down side on a mountain bike. Personal fav is the flow.

I’ve just stuck a 2.35 High Roller on an EX729 – it looks a bit odd 😀

As wide as you can for the weight you want. Stans Flow are 21mm (?) wide and a good weight. TBH I’ve gone back from DT5.1 (21 wide) to 19mm wide and stopped using Big Earl 2.5 tyres. They’re lovely, but overkill for my riding.