Plotinus, suppose you had strong evidence that an all-powerful, infinite creator existed. What assumptions would you make about its nature? How much more plausible would the Abrahamic religions become, in your view?
This is a hard question to answer because I can't really imagine just knowing such a thing outside any evidential context. I suppose you're imagining that, say, I realised that the ontological argument actually works, or something like that. Clearly the Abrahamic religions, or any other monotheistic religion, would become more plausible as a result, because a major claim of theirs would be true - just as, if I knew that the Loch Ness Monster actually existed, I'd regard the claims of other people to have seen it as more plausible than I would otherwise. It doesn't necessarily follow that any or all of the Abrahamic religions would become
very plausible, though, because there might be good reason to doubt them not connected to God's existence. For example, I find the story of the origins of the Book of Mormon pretty implausible, and I find the Christian doctrine of the incarnation pretty implausible too, and I'd do so even if I knew that God existed.
The mere fact that a being is all-powerful, infinite, and a Creator wouldn't tell me much, I don't think. I'm not sure what "infinite" even means by itself. But I would probably think that, from these attributes alone, together with observation of the world, the Creator is probably not very nice, and therefore not perfect.
If this being is infinitely large, it is presumably everywhere at once and thus likely can do anything it wants and knows everything that happens
That doesn't seem a very good argument to me. You could have an infinitely large rock, occupying all space, but it wouldn't know anything or be able to do anything. Size has nothing to do with power, really, or at least so she said.
Besides, God's omnipresence is usually seen not as meaning that he's literally located everywhere, because he's non-material and therefore not really located anywhere - it's usually seen as meaning that he's equally free to act everywhere. God isn't literally "big". And being equally free to act everywhere doesn't necessarily mean he's omnipotent, either, because his ability to act in any given location could be limited but not zero, as with the God of Open Theism.