There's a form of magic from other rpg, litterature and film I can't seem to find out how works in Ars Magica. There isnt' any magical limit against it, as far as I can see. It's the magic that manipulates spaces. Examples:
-The small pouch that can carry hundreds of pounds and has space for everything the Magus would need for his travels.
-The door in a ships wooden hull that seemingly leads to nowhere, but if you open it, you step into a large wooden hall.
-The small tent that can house thirty people.
All these are things that can move around, so they're not portals as such. Would these work in Ars Magica? If so, how?
I don't think this is withion the scope of Ars Magica's magic, or in accrodance with medieval paradigm. Space isn't anything tangible or something you can sense. It is merely the absence of other things. Perhaps faerie magic can do this. But I think Magic can't - you'd need to make the objects smaller rather then the space larger. And IMHO such a space enhancer is entirely too much D&D for me.
I played in a ArM D&D hybrid recently. We set the bigger on the inside effect as being a Muto effect on the container akin to "alter in a highly unnatural way". Although a separate effect was still needed to reduce weight. But I pretty sure I wouldn't want to see it used in strait ars magica. What with the medieval paradigm and all
Why would you need another spell to reduce the weight? Smaller things are lighter than large things afterall. Logic. So the Muto spell that makes something smaller would make it weigh less as well (IMO).
PeTe effects to destroy the volume and mass properties of the things carried --- assuming that the things are solids (PeAq if they are liquids, I suppose).
Alternatively, you could just put Arcane Connections to things in a normal pouch. Then use the ACs to teleport the things to you, when you need them.
Sounds like a Regio to me. No reason why the entrance to a regio has to be fixed. It's beyond vanilla Hermetic magic to create a regio, of course, but not beyond Hermetic magic to find a regio that has this sort of entrance (if one happens to exist).
Alternatively, the Door could be an item that contains a fixed Arcane Connection to the hall, and an effect that teleports people passing through the door into the hall (sort of like a Leap of Homecoming).
Again, this could work like the door (either a regio, or a teleportation effect). Alternatively, the tent could be an item that shrinks the size of people inside it.
Noble's Parma on the "Magi of Hermes" spell Callen's referring to, but the idea seems pretty straightforward to me.
Glamour allows the wizard to create illusions or transform targets into illusions, that have a real physical substance. It's a simple matter to create an illusion of large space inside a smaller one. Therefore, It should be possible for a magus with access to that virtue to generate a functional Glamour of the same image.
Suppose you have a chest. Outside, it's made of wood. Inside, it's lined with finely chiselled bronze. You can use "standard" MuIm to change the appearance of the bronze lining so that it looks larger. Alternatively, you can use a Glamour MuIm to make it physically larger - in particular, it will be able to hold many more objects. However, it will also occupy more space, and tear apart the outer wooden layer of the chest.
If you use Glamour you can no more prevent the reality of the inside layer of the chest from affecting the outside world than if you use a plain MuTe on it.
The real advantage of the Glamour mystery (and it's a big one) is that if you stick to Creo and Muto you only need a single Form (Imaginem) for all your physical needs, instead of needing Corpus, Animal, Terram, Auram etc.
Don't think of it as two different things. You're thinking of it as changing the whole size of one thing and trying to hold it in another. Instead, I change the appearance of a bag so that it appears bigger on the inside than the space it actually takes up (outside). Now the inside is bigger without taking up more space outside. It's also fairly easy to imagine such an appearance: think of having a mirrored (flat) wall of a room; the room will appear to be twice as large on the inside as on the outside.
Even if you don't accept our explanations, it is canon.