The collar is a standard action to activate and the creature immediately increases in size.
Demonic Bulk, whenever you enter bloodrage (which is a free action) you immediately increase in size.
Do keep in mind size change affects cannot be stacked, if that is relevant to this situation.
Normally an enlarge person spells takes a full-round to cast. I'm curious how long it takes in these two different circumstances.
Equipment:
Dire Collar:This leather collar resizes to fit almost any size of creature. Once per day on command, the creature that fastened the dire collar to the animal can cause that animal to grow larger and more bestial. The collar's fastener must be within 30 feet of the animal to do so. The animal gains the benefits of an enlarge person spell (despite the normal targeting restrictions of that spell) for 1 minute.I am thinking as far as with an animal companion, Once activated (standard action) does it trigger a full round action to activate or is it instantly bigger?
and
the Bloodrager lvl 4 ability:
Demonic Bulk (Su): At 4th level, when entering a bloodrage, you can choose to grow one size category larger than your base size (as enlarge person) even if you aren’t humanoid.
When the bloodrager rages does he instantly become bigger?
Remember that it only references Enlarge Person in regards to effects. Casting Time is irrelevant to the effects of a spell unless it's mentioned in the description of the spell itself.
In other words, it only takes enough time to activate the item/ability. The spell takes a Full Round Action. Drinking a Potion of Enlarge Person is a Standard Action (and the effects work immediately). The other abilities that you mentioned, only require the amount of time to activate them (Standard Action for the item, Free Action for the ability).
Note that Enlarge Person has a One Round casting time. It takes more than a "full round" to cast. Those are loaded keywords. Big distinction in this case.