Sotavasara.net

Size chart:
0: Swarms - models of this size never block line of sight or give cover.
1: Infantry, War Beasts, War Machines
2: Cavalry, Monstrous Infantry/Cavalry/Beasts, Chariots, Unique without the Large Target rule
3: Monsters, all Large Targets

Line of sight is always checked virtually, drawn "base to base". Models see in their front 1/4 zone. Line of Sight is blocked by models that are equal to or larger than both: the model whose line of sight we are checking and its target.

Terrain
Hills are size two for each level (so a 3-step hill is size 6) and block sight / give hard cover just like models of that size. Models standing on hills add the hill size to their own size. Buildings and large rocks always block line of sight completely. Forests do not block line of sight, but soft cover applies to all shots fired through or into (but not out of) forests. Ruins do not block line of sight, but hard cover applies to all shots fired into (but not through/out of) ruins. Units standing behind obstacles gain soft (wood) or hard (stone) cover as per the RB.

Cover from intervening units/terrain
Cover is granted when at least 50% of models in target unit are fully or partially obscurred by other units/terrain from the shooting model. In order for intervening models to grant cover, their size must be at least: one point smaller than both the target unit and the shooting model.

Skirmishers
Skirmishers never block LoS, and are seen in accordance with their size on the chart, but provide cover as per the above rules. The gap between skirmishers is treated as part of the unit when determining if skirmishers give cover to something behind them. Skirmishers formed up for combat block Line of Sight like normal units.