how should I tag very long/wide benches & wide stairs?

Sorry if I wasn't clear, I'm speaking of benches, a lot of them are mapped as ways. No problem, as long as you can see them on imagery, I guess.

Have a look at Europe's longest bench : https://www.openstreetmap.org/way/313487068 (not rendered though, such a pity ;) ).

For steps I've seen the debate few times, but no consensus yet. I guess areas is the only solution, but routers won't like it (like for pedestrian plaza).

I find strange to tag the same thing for walking and for sitting... (I have no problem sitting on stairs, that's not my point)

For me on the image, the long stone "shelves", are stairs only. They are too small to be benches (or bleachers?).

I thought the question was about the long wooden thing, behind the statue, which I would call a bench.

My two cents.

This depends on whether the "and" in "long/wide benches & wide stairs" is together and separating. Nevertheless, he mentioned "some benches are between 5 & 10 meters long".

About the stairs, problem with leisure=bleachers would be that it includes more than stair-shaped seating. The most common original meaning of bleachers seems to be those ladder-looking stands not necessarily able to walked over as stairs. See 1st photo and 2nd row of Tag:leisure=bleachers#Examples.

Stairs designed to be sat on are often referred to as benches, and there are really many meanings to a "bench".

It seems reasonable to tag a stair that is intended to be sat on as a bench as well.

the wooden thing at the top is seating, the rest are stairs.

these are definitely not bleachers but actual stairs in a city plaza

I of course don't want to tag both as one object, am just wondering about these who as objects, I am wondering how to tag the long bench, map,tag the stairs. I have seen many people sit on these stairs, but really they are stairs.

Because it's not documented or used anywhere ?

Actually capacity is used, but it's complicated to determine IMHO.

For length, I don't see the point, we use a geographical database where distances are stored as different nodes linked by ways.