Same hole sizes when using Stirrups and posts?

[Williamstown]

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?

[rat52]

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.