Speaking from past experience there are several formats that work very well in this type of setting.

One is the blind map, IE you have a pre made map with each zone labeled and each player records their moves privately and a judge reads over the moves and declares when a conflict occurs.

Two is banners are known and moves made out in the open, suggesting each army can some how scry/spy on each others movements. This can become kinda gridlocked quickly as hot spots will see a swarm of banners heading to a key point.

As for size, playing a small 200 to 500 warband game before the actual game can often be fun and lead to key edges that normally an army would not get.

You can also make banners smaller or large if they have units supporting or interrupting their supplies via being close to or in contact but not fighting a banner.

Example: 1 banner is supported by a friendly banner that is in a space behind it. That banner can have more points added to its roster or some other edge.

Example: 1 banner is within 1 or 2 spaces of a unengaged enemy banner. That banner can have points reduced from its roster or some other disadvantage.

These are just some of the ways I have played out campaigns in the past. If anyone playing or running this needs a hand let me know and I will share whatever knowledge I have.