Well, it allows a Will save to negate the effect if the targeted item is not unattended.Even though it's not 1st level, Shrink Item does not mention requiring the target to be unattended or describe anything about what would happen if some force would interfere with the spell's magic.
To the contrary it _would_ require houseruling to deal any damage at all.Without any house ruling this spell effectively allows a level 5 wizard to instantly kill someone with just a touch attack.
Why should it instantly kill someone?
Also note, that FAQ answers exist for similar scenarios (trying to use Feather Tokens to crush creatures). What actually happens is that nothing (bad) happens.