Is there anything that prevents a character from using the free resize-function of the Enchant Magic Item ritual to store an infinite amount of magic items in his backpack by making them exceptionally small?
As written, that only seems to work for magic armor (emphasis mine):
You can also use this ritual to resize magic armor
So while it could be abused, it's not as bad as it looks.
I doubt it for the following reason.
Bags of holding and handy haversacks are incredibly common in the D&D 4e world. They allow you to hold a very large amount of items and it only weigh a pound.
If you can abstract something one way why not abstract it another way.
Both of these methods are to prevent unnecessary book keeping.
However, I have a feeling that the method suggested in this question could be an interesting opportunity for the DM.
Player: "I'd like to resize my miniscule full blade"
DM: "Roll to see if you can find it"
Player: "Uh Oh. I rolled a 1"
DM: "You've managed to lose every single item in your backpack that was sized below tiny"