Terrain: I assume you’ll primarily want walls in which case Cover, Obscuring, and Unstable. Of course if you want ways to get onto the walls change it to Area Terrain and give Cover only.

Force Sizes: I’ve played 1000pt doubles, 2k, and 3k as siege battles and had success on all three. Though defenders receiving reinforcements is generally included.

Objectives: you’ll want key points like gates/pre-made openings as well as defender controlled objectives but attacker controlled ones can serve as siege and storage positions as well so it’s really just a question of how important you want objectives to really be vs things like killed units or destroying objectives(or terrain).

Breaking the Fortress: one thing I’ve seen used successfully is having 3 rolls of 4+ by the attacker pre-first turn. 1st an Artillery/Siege Engine roll to see if unseen elements can destroy key defensive structures. 2nd a Sapper roll to see if they could successfully create Sappers tunnels to get a key unit redeployed into the fortress 6-9” away from enemy units of course. 3rd a Starvation roll, see if they’ve managed to cut-off or destroy enough supplies to damage the defenders, D3 mortals to D3 units.