Weight and Cessna's
I weigh in the order of 15 stone and instruct on C150's (amongst other things).
I dont normally have a problem with the weight and balance provided the student isnt too big and the fuel isnt full.
Off the top of my head I think I can fly comfortably with a 13/14 stone person and enough fuel and reserves for a decent length lesson. Of course it can get a bit cosy sometimes!. I normally recommend to the larger students to fly a larger aircraft anyway.
Had to draw the line today at full fuel and 19 stone student.. fortunately one of our instructors is extremely slim!
I dont see why you should have trouble getting work with your size, it hasnt stopped me instructing on PA38's, C150's and the like.
Loony