Just how big can a super-Earth get while staying 'habitable'?

I don't understand why large planets with thick atmospheres are being ruled out, even if they have surface conditions that provide liquid water. There is life deep in our oceans, at very high pressures and almost no light. It may not (or may have) evolved there, we really don't know where or how life first started on Earth. It may have started more than once in more than one place, for all we know, now.

On the other hand, I wonder if a planet approximately like Earth needs a large moon to be habitable for a long time, It seems that plate tectonics and strong magnetic fields are important to life on Earth, and having a large moon to tidally drive our mantel seems like it might be a significant help.
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