This may be a question with a fairly obvious answer, but if reduce person or some other polymorph spell is used to make a target smaller, and then said target is placed in a box sized to its new dimensions, what occurs when the spell ends.
I would assume if the box was wood or some other such material there might be a strength check to break out, but what if the material is very strong or if that target fails the check?
Blood squirts out of the seams of the box and the character is reduced to a dead lump of compacted flesh.
Or, at least, that's how my group plays it.
Shunted out, take damage. Or... Just a spray of gore.
Presumably the same thing as Enlarge Person when it's initially cast. Can't kill you, can trap you.
If you're wanting to be grim, gritty and mean about it, you could treat it like a Called Shot to every body location...
http://www.d20pfsrd.com/gamemastering/other-rules/called-shots
He's dead jim
Well depends on the item he is in. If he is in something that is breakable then he breaks it and takes damage. If he is encased that will not break and is not flexible, well the spell will not magically continue as someone previously suggested with enlarge person, the bones in his body will break and his skin will be punctured and the result will be a filled container of goo.