I assume size is based on number of units coming into the battle, not on changing with summoned units, even ones autosummoned on the first turn. I don't think the handful of bodyguards one side can bring to an assassination are enough to bump the size up.
Even less so with Arena battles.
I'm not sure of the size there, but you wish for an arena fight using the Debug mod. I expect I'll be experimenting with that this weekend.
