Adding sink/vanity to Toilet only bathroom

[Christopher Boggs]

I am flipping this house and the first floor bedroom has a toilet only bathroom. It has a pretty big closet next to it, so I am going to steal some of the closet in order to make it a real half bath. I already did the demo so currently the whole wall is hollow. I am trying to decide how big to make the bathroom, and so I need to decide how much room to put between the toilet and the vanity.

Picture - http://imgur.com/o5H3Jri

In the picture the toilet room is in the door to the right, and that whole wall is demoed and hollow. I am going to have to put up a new wall in there dividing the new bathroom and reduced size closet, but I need help deciding how to split up the space.

Thoughts? Advice?