Most ordnance weapons use the large Blast marker, which is 5" in diameter. Any exceptions are detailed in the appropriate army's codex.
When placing the large Blast marker over the target unit, you may find that models from other units are caught in the blast. These are also attacked. However, when initially placing the marker, you must place it to get the maximum number of models from the target unit under the marker. As with non-ordnance weapons, any casualties inflicted on a unit do not have to be selected from models under the marker, the choice is up to the owning player. When firing at a vehicle or vehicle squadron, the hole in the middle of the template must be over at least one vehicle in the target unit.
Next, check that the target is within range and that there is a valid line of fire to it. If the marker has been placed out of range or so that the hole in the middle of the marker is not visible from the firing vehicle then the shot misses automatically, and no harm is done.
If the target is in range and can be seen the massive blast of an ordnance weapon means, it's going to be very hard to miss completely. Nonetheless, the shot may not land exactly where it was intended to. Roll the Scatter dice and a D6 to see where the shot lands. If the Scatter dice rolls a HIT symbol, the shot lands on target. If an arrow is rolled, the marker is shifted in the direction shown by the arrow the number of inches indicated on the D6.
Any models which are entirely under the Blast marker will be hit automatically. Roll a D6 for models that are partially under the marker: on a 1-3 they are unharmed, on a 4-6 they are hit by the full force of the blast. Vehicles will be hit regardless of whether they are fully or partially under the Blast marker.
EO Note: Due to the use of modern minis, if you're using bases bigger than 25mm for standard infantry, be generous for what counts as partial or full hits.
Resolve Ordnance hits with the Strength and Armour Piercing values on the Ordnance table.