Several years (and versions) ago, I asked one of the developers. At that time they said, "There is no size limit. Other than how much memory is available." I knew that probably couldn't be right but couldn't get any other answer.
And don't know if that still holds for current versions.
Take this with a grain of salt.
As far as I remember, size of variable while running flow (on memory on central and RAS) is very high. But less data is written to DB (value truncated), so it cannot be seen full value from past runs.
Also found some operatiosn (like ssh shell) truncate data for large outputs. So be ready to use alterntives like redirect to temp files, split on more steps, pre filter data, ...