i have a 5x5 16v16 summer duel which is perfect for playtesting 32 player.
125 unitcap (share with all on death), fullshare, the smallest maps and a strong local player base is essential for hosting the best large lobbies. 125/5x5 does make international 16player matches viable. 20x20 16 player 500 unitcap lags fairly reliably, any more map or units makes the whole thing fall over. the amount of idiots who insist on hosting massive lobbies on large maps with large unit caps is the reason why large lobbies get such hate. 125 units is very good. 250 is decent. any more an the unit cap expands too quickly as people die and early spam is too easy.
implementing all measures to mitigate lag certainly gives the lobby the best chance of success. the reality is that 8v8 with 125 units is 1k per side. 16x16 brings it up to a grand total of 4k maximum units on the map. on a tiny map thats really a lot of units. when you do hit your unit cap because spam, then it is easy to give it to your allies, along with static defences etc. and carry on spamming. team work is most crucial and appreciated in 8v8. full voice chat makes the whole experience shine. 32 player lobbies means you don't have to kick the few people who showed up at the last minute.
full share ensures when someone drops, the game just carries on. people do complain about fullshare for long list of reasons. but its all mute because 16/32 player is not the meta the fullshare detractors normally play so their opinion is irrelevant. on more competitive maps with sane numbers of players, share until death makes perfect sense.
Tangentially i'd love to see the return of "Share after death" the status quo pre Fullshare/ShareUntilDeath. additionally LOUD also has a mode where you have to kill all the ACUs AND SACUs. maybe not the best idea for 32 player but lmao id try
sharing armies does indeed sound fun. i would like to try a 16 player and 32player match with shared armies 
tl;dr
the main factors for good large lobbies are
player count
map size
unitcap
relative player connections.
the higher the first value, the lower the other 3 values must be.
