If you mean Largest in terms of number of combatants, I've rarely had opponents outnumbering players by more then 3 to 1, usually it is even lower, especially if the player's characters are on the white hat/investigator/primitive screwhead level. Often if an equal number of opponents attacks the players, the heroes end up taking out about 1/2 to 2/3 of the opponents with the white hats ganging up on the rest.

If you mean Largest in terms of power and size of things they've fought, I guess a Minotaur Demon made of living lava with volcano powers was about the largest most powerful thing I've ever thrown at players.

In terms of length of cinematic combats I'd say you should measure it out according to an idea of screen time. Every fight should have a purpose, even if it's just to let your players blow off some steam and get their juices flowing. Usually an equal number of mixed power level players and opponents who are more powerful then human (like Vampires) lasts about 15-30 minutes. I tend to try and keep things fast paced, cinematic, and exciting. For Episodic style play you can even run quick teaser combats that only last a round or two, if you start things in media res, and with the bad guys low on life points. Beat up the bad guys, say something pithy, throw in a twist, and bang straight to the credits sequence, and then the rest of the episode.

For lieutenants and big bad type fights, and episode, or season enders things can be drawn out. Drama points on both sides can be spent, earned, and re-spent. For cinematic style roleplaying the GM should cheat to make such fights dramatically appropriate. Nerf the bad guys, or give them some bigger teeth, raise or lower the level of Life Points, and dont forget to use Drama Points for the big bads. This advice also applies to the low level bads, but it is particularly important for the big bads and the players fights with them.

Also given the quick sheet, ability score, no rolling dice rules for bad guys in the Cinematic Unisystem it is very simple to run multiple bad guys, and indeed multiple types of bad guys in big climactic combats. Let the lesser heroes take on a bunch of guys, while the big hero dukes it out with the big bad. Or switch it up and have the big hero mow through an army of disposable villains, while the lesser heroes gang up on the big bad with a kick ass plan.

For some good info on running really big battles scenes, and mass combats with the Cinematic Unisystem you should definitely check out the Army of Darkness RPG, which has some good info on this topic.
Morgan's Unisystem RPG Page
www-personal.umich.edu/~mcellis/Eden/