How do I make a box/wagon/room bigger on the inside?
Alternately, how can I get my entire lab/bedroom/castle to fit inside a tent/chest/trunk/other portable object?
In a related question, how can I make a tent that, when pitched, provides an opulent interior environment that will truly impress my visitors?
Off the top of my head I would guess only way to do it is to somehow create some sort of regio and entrance to your room/tent is simply doorway to that.
It's actually rather difficult to do hermetically.
A previous discusson can be found here: https://forum.atlas-games.com/t/magic-of-spaces/4677/1
Make the box your laboratory, give it the free virtue boundless, and when anyone asks, explain you have no idea how or why the box has endless space inside, but you are keeping it.
Of course the ST is going to come along and ruin it. They always do, but often in the most exciting and wonderfully told ways. 
If you're a Merinita, initiate and use the Glamour Mystery.
If you're not, you're toast
Brilliant! Guest Who...
I suppose that there are two ways and half: Glamour Magic, Faerie Methods and Powers, and utterly one Rego Terram to teleport and to be one any place all time. Then the enchantment should be the door (the room) and the Blue Box... i mean the TARDIS... I mean the wagon or something like that.
It qualifies as a change that makes the result a "highly unnatural" object. Check the corebook for spell guidelines such as:
"Muto Terram level 4 -- Change [an object made of] earth so that it is highly unnatural." This is not nearly as helpful as it sounds, however: if your magical belt pouch is so large on the inside that you managed to fit a castle into it... it's going to be a very heavy belt pouch!
Again, use Muto -- this time to shrink the (would-be) contents; ideally, this is an effect enchanted into the tent/chest/trunk that automatically affects the (would-be) contents. If you want it to affect a variety of non-living items, use Muto Terram + requisites (typically Animal and Herbam). One of the most common redcap items in my saga is a red cap enchanted with this effect. Note that this is better than simply making the container larger (see above), in that it makes the transported objects lighter.
If all you care about is "impressive", Imaginem is probably the best choice: Muto Imaginem Level 30, Base 5 (change an object to all five senses) +1 Touch, +2 Sun, +2 Room.
So that's.....
Muto Terram
Base 4 - Change object, highly unnatural
+1 Touch
+2 Sun
+2 Room
+2 metals, gemstones
+1 Herbam
+1 Animal
+3 lvls environment
+1lvl 2 uses/day
Total: 49
one way that I see to do it is use hermetic architecture to make the box a doorway to a regio
Aren't there some rules for creating a regio? I think to remember something in ROP:M ...
That would be also a nice solution.
Salvete
Widewitt