Do we have an equivalent for Persian's proverb "to stretch one's leg more than one's rug"?
Bite off more than one can chew
comes to mind
If you want to tell someone not to overreach, you can say
Don't bite off more than you can chew
Don't let your reach exceed your grasp.
Over-extending oneself happens when we go beyond our limitations.
Over-reaching is when you over-step your boundaries or "circle of authorities".