If you want/need to create a hard, definitive rule, then look to one or more of these possible standards of measure...
Is it an "innate part" of the creature w/ MR? (the mage, the hag, whoever)
Was it included during the Parma ritual?
A specific distance - 1 pace or "arm's length" or something
Arcane Connection/association - is the item usually in possession of the person?
Simple possession - if they're wearing it, and/or if they're holding it and can lift it (in 1 hand?), it's good
Hard size and/or weight limit - per item, and/or overall load
Limitations - no additional live creatures (larger than insects), etc etc
There are too many variables to "tell" you what you should do, the most important of which is your Troupe, whom only you, as SG, know well enough to decide what will work best.