The bigger, the better of course! That is, if you have the space to spare. You also have to consider if it is to be designed for one OR two users? Do you want to provide for a seating area or central island feature? Is it a pass-through closet to the master bath? These are all things to consider. Minimum for a nice walk-in would be 10'x10'. Closer to 12' x 15' or 20' would be ideal depending on the context and needs of the space & end-user.