I'm completely new to the world of 300BO but am entertaining the idea of getting into it and so have been doing some research on parts.  However I've got a gas port question: is there an ideal port size/measurement that will allow a gun to reliably run suppers and subs (both suppressed and unsuppressed) without using an AGB?
I may be asking to have my cake and eat it too, so forgive me if this is a dumb question.

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There really isn't a one size fits all answer. It's going to depend on which ammo. If you reload you can make just about any port size work with proper powder selection and load workup.

Just buy a reputable, high quality barrel and go from there.

I'd recommend Noveske, Seekins, LaRue, DD, V7, something along those lines.

I have a Seekins. It absolutely runs subs and supers equally well, with no AGB.

Would you be chance happen to know the port size on the Seekins barrel you have?

And I don't do any reloading unfortunately, so tailoring a load to the gun is not an option for me.

No idea. They don't publish it either. A lot of companies keep those figures obscured. Not that you can't find it out, but I never measured.

Sounds like you have an excuse to start reloading. Getting into 300 BLK is like buying a pickup and never using the bed. You're losing out on a lot of the fun and flexibility the round provides. It's not expensive to start reloading either. For subs a simple press will pay for itself in the first thousand rounds.

Depending on your barrel length it's possible.
I have a 9" AAC barrel with a port sized .100"
It cycles and locks back in all four ways with a carbine buffer.
Suppressed supers
Suppressed subs
And both unsuppressed, they all lock back.

Nice! This would be my ideal situation.