Anything that fits in ram, at least in the context of "a table" in a RDBM probably counts as 'micro data'...
Either you don't care about that query's runtime (a perfectly valid approach if your use case is "this creates a report after COB on Friday that needs to have completed before 9am Monday morning"), or you need someone who knows what they're doing take over from you (also a valid approach if this is in your tech demo or POC, and you're the Technical Founder who's now out of their depth and needs a proper engineer to build a scalable product for you).
I don’t know about “big” or “small”, but the threshold for “done properly” has to be really low for that to become a bottleneck.
Most people today don't have the computing tools that you do. Yes, a consumer laptop running Numpy can process it quickly. But they have Excel, not Numpy...and Excel cannot process millions of rows. So in the context of the tools they have, it is a large table.
So Big Data services could be just as useful to a small company with a mere 100GB of data once they learn how to squeeze the juice from the berries they've collected.