Along with timing, scale is one of the building blocks of a successful effect. The scale of an effect is different from the size of an effect - it is important to make this distinction. Scale is the relationship between the artistic detail of the effect and the game world; the size of an effect is the direct space in game units that the effect occupies.
For example, using particle art for stormy clouds in the sky as a base for cigarette smoke would result in incorrect scale. The particle size can be shrunk to fit the size of cigarette smoke but the detail within the artwork will be much too fine. This is similar to issues in game world texturing, where a wall might have a brick texture where the bricks are too large or small relative to the scale of the game world. Another influence on the perception of scale is the timing and duration of the effect. A firecloud from a large explosion will last longer and travel further than a smaller explosion.
The primary function of size is to communicate the affected area of the design. The player needs to know if a specific game area will be affected by damage, healing, protection or something else. The size of an effect also needs to meet gameplay requirements by not being so large as to obscure the player’s vision unless that is what is called for by the design. Visual noise in the scene can confuse the player by competing for their attention if the size or scale is inappropriate.