There is an example in the construct community discord, where Mikal showed a way to find the breakpoint if the text overflows.
Tbh there is no real great way to solve that currently, only workarounds. There was a suggestion for a feature like that done a few days ago https://construct3-21h2.ideas.aha.io/ideas/C321H2-I-193
I would suggest you to just design your dialogue so you don't have large text that will overflow, instead already break them up when writing them. This will save you alot of hassle