depends on what you mean by average level. Depends on how you want people to play it - i.e. what tactics will be used, how they will carry out objectives, where they will take cover, what weapons will be used in what areas... lots of stuff
Look at some indoor maps (like spooger's RS maps They have a good layout for an average player count, nicely structured with sufficiant cover. Along with archives, they're my favourite indoor INF maps. Now imagine if they were expanded over a huge area the size of Extreme prejudice. It would be too big, like a huge maze, you'd never find anyone. It works the other way too, an outdoor level with limited size (only as large as Texasbank or MWH) would result in rounds ending within a minute of respawning.
That's why its important to plan or work out how your level will look and play, once you know that, the ideal size will become more obvious.
I once ate a pie with a plastic fork.