The Ars Magica game includes detailed spell design guidelines system, based on a guidelines to set the Base Level, which is altered based on the three spell parameters, size, and possibly other modifiers. While the core of this system is given in the core rulebook, some options and details are present in supplements. The following page collects all of the relevant material from all the supplements, adding to the Definitive Edition text as relevant, and summarizing the entire system in one place.

Summary

Spell Parameters

First determine the Base Level and, hence, magnitude of the effect using the specific spell guidelines for the Technique-Form combination. The spell must abide by the Limits of Magic.

Standard Hermetic magic can make use of the following spell parameters, which may add magnitudes.

Hermetic Spell Parameters
Additional Magnitudes	Range	Duration	Object	Container	Sense
0	Personal	Momentary	Individual	Circle	Taste
+1	Touch / Eye	Concentration / Diameter	Part	-	Touch
+2	Voice	Sun / Ring	Group	Room	Smell
+3	Sight	Moon	-	Structure	Hearing
+4	Arcane Connection	Year (R)	-	Boundary (R)	Vision

R - Spell must be a ritual.

Ritual required for Momentary Creo spell creating a lasting thing, or spectacular effect, or level above 50.
Ritual spells must have level 20 minimum.

Formulaic or Ritual spells can be invented with Ranges, Durations or Targets that are not listed here. This is usually slightly more difficult than if the closest category were used, but is left to troupe interpretation (see Wind at the Back, The Bountiful Feast, Trackless Step, The Earth Split Asunder).

Mysteries can provide further spell parameters. In particular, Merinitia magi have access to the following spell parameters:

Road Range: +2
Bargain (additional) Duration: +2
Fire Duration: +3
Until Duration: +4 (R)
Year + 1 Duration: +4 (R)
Bloodline Target: +3

Size

Size: Each +1 adds x10 to mass (+3 to Size) or area (x3 diameter).
Base size-change guideline is for x10 size, or x100 shrink. Add another such factor per +1 magnitudes, in addition to the basic magnitudes needed to affect Size.
Intellego spell aren't affected by size.

The basic sizes of Targets are:

Part: as Individual.
Group: Mass of 10 Individuals.
Room: Large enough for 100 Individuals.
Structure: 10 Rooms
Bounday: 100 paces in diameter

Additional Magnitudes

Further modifiers described by the rules or in example spells:

Requisite +0: Necessary for the effect.
Requisite +1/+2: Adds and effect to the spell. Add +2 for 6+ magnitude secondary effect. No requisite needed if only cosmetic.
Additional Effect: +1 (same TeFo)
Special Effect +1: Slightly beyond the spell guideline (see The Silent Vigil)
No Words: +2 (see The Kiss of Death)
No Gestures: +1 (see Black Whisper)
Complex Effect: +1 (see Weight of a Thousand Hells)
Fancy Effect: +1/+2 (see Teeth of the Earth Mother, Creeping Chasm, The Earth Split Asunder)
Craft Magic Flexibility: +1 to +3 (high quality is at least +3)

Wards

The target is the thing protected, and the range is to it.

Wards with a Circle target: Warded things cannot act across the circle, or leave/enter it, or damage it, directly or indirectly.

Craft Magic

Rego craft magic may fail if Finesse fails. Roll Perception + Finesee vs. mundae EF +3 for a day's work; +6 for month; +9 season; +12 year. Mundane EF is 9 for a skilled craftsman's daily work.
Creo always creates a functioning version of what is intended. Control details with +1-3 magnitudes (for Formulaic or Ritual) + Roll Intelligence + Finesse EF 0 for basic details; specific details or multiple basic details is +3; elaborate details EF as Rego craft magic.
A spontanenous un-Finessed Creo spell creates random item of kind; a Formulaic or Ritual spell creates exactly the same item.

Summoning Spirits

Summoning spells bring the spirit instantly to the caster.
The spirit may be summoned directly into a ward cast by the summmoner (if the ward is not cast by the summober, ward prevents entry if it penetrates).
A formulaic or spontaneous spell can summon a spirit in Mythic Europe, including regiones.
A ritual spell is required to summon a spirit in another Realm of Power.

Creo Targets

The target of a Creo spell that creates something is always an Individual or Group. A spell to createa a part of something is Creo or Muto.

Container Targets

Static Target: Affect any valid target within the container at the time of caasting, and continue to affect it even if it leaves.
Dynamic Target: Affect any valid target within the container for the duration, ceasing to affect it if it leaves.
Containers can move if they are still the same thing while, and after, moving.
If the same container still exists, then the target still exists. A circle is a different circle if its size changes, or if it is transferred to a different surface.If a Boundary is nothing more than the thing marking the border, then it is as vulnerable as a circle, and cannot be moved, altered, or expanded.

Magical Senses

The range of the Intellego Form (?) spell is the range to the recipient.
Granting a magical sense to many at once is a Muto Mentem spell, with an Intellego Form requisite.
Magical senses must penetrate the Magic Resistance of creatures sensed (record the Penetration), as well as the Magic Resistance (if any) of the person granted the sense.