Optimum map size for mvm?

As big as it needs to be.

Map size is largely dependent on what kinda style of MvM are you going for, in my opinion:

• The launch maps are fairly small (Engineerbots did not exist yet) and have a lot of places for players to either hold out, reset the bomb or delay the bomb runner by knocking them into a pit (like Mannworks or Decoy have)
• Bigrock is essentially the biggest map MvM has (and is essentially what Freyja's map is in terms of scale), so the waves need to have a lot of push to them, so the bots have a legit chance to deploy the bomb and the Engineerbots get to do their job of being horrible (Decoy also has Engiebots because Mecha Engine needed a third mission for it)
• 2cities maps are kinda in between of the earlier two: Fairly long but have different methods for closing the distance (Rottenburg has Engineerbots that build teleporters, Mannhattan has the gates)

The only thing that the map size is going to affect is how the waves need to be designed (see Bigrock's advanced missions) so the players either never get pushed out of the front or get pushed all the way to the hatch. Sometimes you can do this with a couple (or a lot) of Tanks/Giants or just sprinkling some crits on bots to give them some muscle.