I would say a 5' x 7' closet would be a good size without being overkill.

Somewhere between 5'x6' and 5'x7' works really well. Don't forget the option of putting in double stacked hanging racks vs just a single hanging rack. With this size closet, you should be able to get plenty of hanging space + a nice size column of shelves/drawers for folded items. I would not go any narrower than 5' to allow plenty of walkway space in the closet.

Our kids' closets are back to back measuring out 5' x ~7'. One of them has the door on the 5' end and the other has the door on the ~7' side. The one that has the door on the ~7' side feels more spacious than the one with the door on the 5' side even though they are the same size. Just an observation you may want to consider as well during your planning.

Hope this helps!

Our kids' walk-in closets are 6 feet wide. They are 39 inches deep. The door is centered with hanging racks on either side. We put shelves below the poles on one side of each closet. In the boys' closet we put double hanging racks for shirts/jackets.

In the boys we put a coat rack. We did have dressers for the kids though. One drawer was socks, one underwear, one shirts, one shorts, etc. I think there was a time when I "matched" pants & shirts together. BTW, two boys shared a room and my dd had her own.

In the Master we have 2 closets of the same size. Dh's has 2 double racks; I have double only on one side. On the back wall, our own have shoe boxes. I also have hooks for camera bags and backpacks.