I would guess that 23.5" might be fine for someone 6' but you probably don't want to go any smaller. Since you can always raise the seat post within reason, it's the forward reach that really matters. Stem length then is the biggest factor, and easily changeable. As examples I present my two bikes, a UO8 and a Gran Sport, both from the early 70's and 23.5" (or 59cm, more or less). I swapped the Gran Sport's stem for one a cm shorter and it made the bike much more comfortable.

But then, there was a time when I thought maybe I'd bought the UO8 too big. My back would get tired from the forward reach. But as I've ridden more lately (and according to my doctor, shrunk an inch or more) I've felt very comfortable. Back fatigue is not a problem so I can probably handle a reach even greater. I've commuted about 800 miles on the UO8 this summer and it doesn't feel too big at all.

The point being, your thoughts on fit and frame size may evolve with your riding.