what is the smallest functional walk in closet one can build?
We did one about 5' by 4' with clothes on three sides with door entering on center of one side. This left about 2x2' in the center on one side to access the clothes etc. :)
Or, you can run a walk-in with a rod/shelf down only one side. Door swings against the wall. On that door swing wall you can still get some storage for things that hang flat...like belts and ties if ne1 still wears them.
Figure 2' for the rod/shelf, 2' for the door and a little more. Depends upon what you've got to work with.
Some of the DIY closet companies that are sold at HD like Closet Maid...can't remember the other one....have web sites with applications that will help you. Tell it the space available and it will configure options.