Kit, do you mean make the space inside an object bigger physically?
I'm sensing you are thinking of a Tardis-like effect done via magic. If that is the case, I apologies for bursting your bubble. That is impossible.
Magic is subtle, and doesn't break the laws of the universe. Hense why time-travel, physical shape-shifiting, and conjuring fireballs is all in the realm of impossibility.
I would recommend you do some reading on what magic is and is not, and what it can do and what it can't.
If this is not the case, please do elaborate.