If the concern is actually flaw points, then let's look at that.
Highest flaw limits:
Cannot target humanoids: -5
Seen on spells like Hold Person or Charm Person. Note: The flaw has to be an actual restriction on the spell. Only being able to target humanoids is a restriction. Only being able to target non-humanoids is not, given the humanoid-centric nature of society). Restrictions designed to gain the benefits of other powers (eg: Careful Spell metamagic) are not actually flaws.
Dispersed by a moderate wind: -4
Being dispellable by a common environmental condition is a large flaw. Requiring something more significant and rare, like a fire or a strong wind (likely only available via magic, except under rare conditions) are lesser flaws (-2 for fire, -1 for a strong wind).
In general, flaws are rare.
Maximum number of flaws in a spell: 3 (Charm Person). A couple others have 2. Most have 0 or 1.
Most flaws involve alternate ways to dispel or end the effect (winds, fire, being harmed, being shaken awake, wearing armor, recasting the spell, etc). In fact, there are only a couple of very specific flaws that are not covered in that scope.
Note: Negative numbers are not themselves flaws. Spell Range can be rebaselined for all spells such that it doesn't use negative numbers; it wouldn't change anything. Targeting selection shouldn't be a flaw, either; you can choose a creature type, or any creature, or objects, or creatures+objects, etc. Since all spells must have a target of some type, it's reasonable to consider this its own property, effectively just like Range. Same for casting time.
If targeting is not in itself a flaw, then Charm Person also only has 2 flaws, the same as the general maximum. As such, we can consider 2 flaws as the maximum allowed.
Hmm... I don't see a single spell with an area that has the area size more than one level below the spell level (so -1). So probably limit it to that.
So, extra rules:
1) Some properties are allowed to have negative numbers. These are not flaws, as they are properties that all spells have. They merely have adjustments on where to start counting, and reasonable defaults that count for 0.
2) A spell with an area of effect cannot have an area size more than one level below the spell level. (Max credit of -1.)
3) A spell cannot have a damage value higher than its spell level.
Caveat: Fireball, Lightning Bolt, and Meteor Storm violate that rule, but they're not my problem.
4) Creature-targeting exclusions should rarely be applied. Rely on resistances and immunities first before considering a targeting exclusion.
5) A spell cannot have more than two flaws.
6) Be cautious of flaws for anything other than dispelling the spell effect.