In our previous residence, I added a walk-in closet. I think it was 4x5'.
Open door, take one step, turn on light, all clothes are within reach yet
far more convenient than a conventional closet and less % corridor than a
larger walk-in. OK, so this was in a singlewide. Build to fit your needs
and your house.

We are now in a 1953 "real house" and I wish I had that closet here. We've
bought six wardrobes to augment the closets.

I agree with the comment about ventilation -- at least make sure the closet
has good air flow. Especially A/C to avoid mold and mildew on the clothes.

I think the bedroom I grew up in was 10x12'. My sisters shared the smaller
bedroom. This was in a very large 1300 sq ft house. It was large because
the available area was used for real living space and the bedrooms were
sized for sleeping only. Fifty-four years after my parents built it, they
are dead but one of my sisters is living in the house, and my other sister
would retire there if it were available.

Edward
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Art Works by Melynda Reid: http://paleo.org