Foreword: I'm posting this here instead of, say, Worldbuilding, because while it is based in a speculative concept, this is a question purely of biology as we know it instead of speculative biology under non-real conditions. I would address that later and elsewhere. And sorry if this is actually more physics than biology, I have trouble pinning these things down.
I'm trying to determine what the maximum height of a non-woody plant stem can be (i.e. I don't want the tallest flowers). I think the primary issue at hand is load-bearing and resistance to bending. So I found this 1997 paper by Schulgasser and Witzum that sort of discusses what I'm looking for. It suggests that the plant's ability to resist shear forces is a matter of how bundles of fiber are arranged inside the stem and of the plant's internal pressure, which affects how much weight it can stably bear. But I'm no biologist, so the paper is difficult to penetrate. I'm sure there are other issues limiting the height of non-woody plants, like the maximum height above the ground that capillary pressure can raise water. Considering all factors, what is the theoretical maximum height of a herbaceous plant?
I've found this question, which concerns the tallest real herbaceous plant, while my question concerns the theoretical. But I would like to extend that question - are there any examples of fossil herbaceous plants that are larger than the modern banana plant, and do they run up against the theoretical maximum?