We've been playing in 15mm on a 4'x6' table and running 20 turns (in good weather) and have found the games about right.We've extended the deployment zones to 12" (at our old Empire basing it works out that 1BW=1", so we tend to just use inches). With the extended game length, we've modified the Army Morale roll from 5 dice to 6. Our reasoning was that the average roll on 5D6 is 17.5, allowing an army at it's break point to stay the full 16 turn game. The average on 6D6 is 21, again allowing an army at it's BP to stay the full 20 turns on an average roll.
We've yet to play a game in mud (still getting comfortable with the game: we only get to play once a month or so), but I'd think extending the game length (and modifying the Army Morale roll to an appropriate number of dice) would be the answer. How many turns to add I'm not sure. I'd probably start with a 50% increase and go from there. Probably the best course would be to just ignore the turn limits for the first time or two until you figure out the best length.
A note on the Army Break roll. We've kept the 4D6 roll if the attacker controls the objective. Makes it a bit more important, especially as the game continues.