Same hole sizes when using Stirrups and posts?

I'm putting in some stirrups to hold my bearers off the ground. Do the same rules apply when using stirrups as compared to posts? For posts I would normally dig a 300 diameter hole (or 300x300) approx 400mm deep. However given the stirrup plates are much smaller I could get it in a smaller hole (I assume the concrete mass should be the same).
I was planning on cementing the stirrup in as opposed to bolting the stirrup to a cement slab/block. How far (deep) into the concrete should I set the base plate of the stirrup?

If there are no uplifting forces
(ie. deck only) then depth doesn't matter but as a general rule I have always placed a couple of bolts in the bottom plate to embed in the concrete.