The room is roughly 14'x16'6". This is roughly a 4' pathway which is ideal in a closet. If it's a smaller room, consider a smaller island, but sacrificing walkway space will make it feel cramped. Hope this helps!

Building a house and the master bath has a lot of floor space in the middle. I want to put a cabinet like this on the outside of the closet in the wide open space. Would it look ok. Can't find any pics with this idea.

Putting an island in the middle of a bathroom may not be as functional and may look out of place. We would suggest using the opportunity to incorporate an interesting floor tile pattern into the space. You can see an example in our Circle Park Place Project (bathroom).
http://www.houzz.com/projects/171413/Circle-Park-Place