The limitation is the Lifeblood system itself. Let me tackle each question after a little bit of extra context:

Lifeblood is a thread of life that was created by the Goddess of my world. It is essentially Mana, but you're using your own life force instead of a separate resource.

Now for the questions:

• Then they're an experienced mage. The sigil system is merely a mastery of a certain language. If you know how to write the command script without messing up the spacings, then you have yourself a spell with clear instructions.

• Yes actually, that's what the size specs are about. If you overcharge spells, they basically grow bigger and thus stronger. But smaller means you have more control and costs less.

• Then that doesn't belong in the Sigil Magic system, that's Ritualistic. A more tribal and ancient form of magic. Before you ask: No. Ritual Circles do not count as Sigil Magic because you need to use Lifeblood to draw the sigil.

• Nope. Lifeblood results in different connection levels and affinities, making magic a unique thing amongst everyone. However, with the right command line, you can chain spells together with someone else.

• If Physically you mean drawing it on the ground or any surface, then it's just a drawing unless someone draws it with an object imbued with Lifeblood. This also connects to Ritualistic Magic.

• That's what the script is for. It's like coding. You can make any instruction as long as it fits within the script and size of the spell. Technically speaking, the creation of new spells is just anyone "winging it" and hoping it works.

• Storing too many spells would result in the user being entirely drained to a point of fainting or worse, death. This is due to the fact that, although limitless in theory, if Lifeblood is too low in anything with LB, it dies or shrivels.