For what you want you dont have to go much bigger I am only guessing as not played that small but increasing the width by 5 or 10 would do it. A bit longer to contact but jockeying for position is a key part of any engagement. Of course you could just limit visibility to 4 or so. That should get you a straight fire fight with limited manuevering.
If you look at the battle you originaly suggested 20x20 = 400 hexes so 20 units a side means 10% of them could contain a unit.
50x50 = 2500 hexes you would need 125 units a side for the same coverage.
80x80 = 6400 hexes so you would need 320 a side.
Had to get the calculator out for this one if played on a max size map for your unit density you would need 1600 units a side
Manuevering to find a weak spot is in my view a key part of the game as you rarely have enough forces for adequate cover of the whole battlefield. If your probe meets stiff resistance you need to be able to switch to plan B though time is the other key factor.
On the flags place them on sensible locations in your path like hills crossroads villages not in some field. Do not use shotgun place in 3-5 groups possibly more on a big map. A force normally has several objectives the main one can have the most flags others are secondary.