The typical urban/suburban "city flock" of 6 birds +/-, all hens (typically, and typically all different breeds) is a hobby flock, or perhaps a vanity flock. Or simply pets. Even so, that's often all the municipality will allow, and no disrespect is intended to those chicken keepers. Glad they can do it, it has benefits for the rest of us.
Small is anything between that and a score of birds - the size where you can reasonably talk about "production" of eggs in quantities useful to a family, rather than an individual/couple.
I have what I consider to be a middlin' to moderate sized flock (in the sig below). As soon as you are buying feed 500# at a time, you are there. I incubate 12 eggs every three weeks, take 1-2 birds each week for table to maintain numbers, produce enough eggs for incubation, my own use, and to sell in quantity that puts me break even (on a good month) for feed costs.
If you are buying feed a ton at a time? Or seriously considering trying to grow your own on acres? That's a large flock. We have a few posters like that, I learn from their comments most eagerly.
***and my views on the subject are probably atypical.