Well, that's sort of up to any group to interpret as they like. There are rules for how big a focus item has to be, depending on how many focus item slots fit in them, but as anything, they are more a guideline than a hard rule. I think mainly, it is a balancing thing, making sure you don't smuggle the magical equivalence of a nuke in a matchbox.
However, there is "focus specialization" available, and one of the effects, I think, is that you can fit more focus item slots on an item. Again, this is up for interpretation whether or not this means the item still needs to be bigger, or if those additional slots allowed by the specialization don't count for the size of the item. There is no WoG on that, as far as I know.