IMHO I think that an instructor needs to know and how to teach aikido and can do that for a duration of the class. As along as those conditions are met and they have something of value to teach then I am happy with it regardless of body shape etc.
On the point of physical fitness, the term is ambiguous. I regularly workout in a gym (cardio, abdominals and weights). My last fitness test indicated that I was above average fitness based on various indicators including resting heart rate, blood pressure, recovery time after vigourous exercise (time taken to return to resting heart rate from pulse rate of 146), body weight to height ratio (body mass index), percentage of body fat, weight lifted vs my weight and flexibility (on the -1, 0 and +1 scale of selected stretches). This level of physical fitness helps my ability to go through class and the mechanics of the movements and techniques but its little value in terms of understanding the principles and concepts of aikido and how to apply them.
So I think its important to be clear about what constitutes "physically fit" if this is an important consideration. What is the measurement standard and why is this appropriate or desirable.
Janet Rosen wrote:
...Maybe what we think an instructor should be is a reflection of our own values. ...
All the best for training