I searched around a bit for this and have come up empty. Is it land, men in his command, wealth, a combination?
This. One of the devs confirmed it the last time this question was asked.Number of holdings, then age as a tiebreaker.
Aside from the part where the baron immediately begins hemmoraging 3 of those baronies, also no.
When I first started as Byzantine emperor first thing I did was conquer Venice. Despite being only a one county duchy in a sea of Greek, Sicilian, and Anatolian duchies he still commanded a powerful vassal rank.
All content DLCs, with Conclave and the lot. Maybe it was viceroyalties vs doge difference? The fact the Doge was a close member of my dynasty?
You can test this out pretty easily too. Just pick byz emperor and de jure Venice right off the bat. Dumno why but he was a powerful vassal for me up until I started to have Kingdom vassals.