Canada

Also what is available in a common/public way.

Some people have to spend a lot on housing just to have a place to have sex without bothering others. Or use a table saw. Or play the trombone. Sew a quilt. Cook a big meal.

If we have accessible, shared locations to do these things, the actual living space doesn't have to be as big.

The newer, fancier condos have all sorts of sports like gyms, swimming, squash etc. And they have function rooms to throw parties. And hanging out space. If that could be generalized to all people it would contribute to reducing the "how much space does each person need" value.

Personality and lifestyle dependent, too.

The problem I have with the space tax idea, which I think OP has mentioned before, is just that it presupposes what people need without much justification, and then applies a penalty to force that outcome. Really, you only want to make people pay for what they take, and people buying big houses definitely do that. (Having inequality in the first place is of course it's own issue)

For sure, a thorough study of what Canadians need would be helpful to something like this. Could inform what a good space to person ratio could be. Especially in Canada.