More than 5

3 or more

I think it depends on a few factors. Ethnicity for one. Place of residence for another.
For example, if I met a Maori family with 4 kids, I'd think that's pretty normal and average size. But, if I met say, a French family with 4 kids I'd say that's a large family.
If I met anyone with 4 kids, regardless of ethnicity, living in a city, I'd think it's a large family. But, if they lived in a country/rural town, I'd say it's average size.

I come from a large family (7 kids), and we were considered large no matter where we lived. But at least in our little rural town we weren't the only ones, and we weren't THAT much bigger than the average family.
I have 4 kids now and live in a capital city, people think we're crazy to have so many kids. But, I wouldn't change them for the world. We're getting pretty claustrophobic here and are looking to move back to our hometown before Christmas. I know that there, no one will bat an eye at us in the supermarket. Or ask if I'm running a daycare. Or assume that we're a blended family.

I agree with the 3+ responses

3 or more

More than 3 or 4 kids I guess. It’s personal interpretation though isn’t it?

Anything 3+

3+

These days it's more than 3 kids, that's when everything changes and you need bigger everything. Before WW2 a big family was probably at least 6 kids because nothing really had to change to keep adding to your family and kids were well and truly out of home by 18 back then.

4+ kids