Spells with the Area tag can potentially affect every target in a circular area. Place a marker on the table where you want the spell to go off. Place the end of a measuring stick of the appropriate length on that spot to determine who is caught in the radius. Every character whose base is even partially within the area is hit by the spell and may be affected.
Does this mean a short area spell has a radius of 75mm, a diameter of 150mm, or is the diameter 75mm, as the cone is wide. Potentially a spellcaster could nail themselves, or you could end up with a 360mm circle of terror on the board.