Equation Editor- problem when editing an equation
 

I use Equation Editor a great deal, and I constantly have the same problem.
If I create an equation, and later go back to edit it, the font and spacing
of the equation change. I try to resize the equation by enlarging or
decreasing the size of the window, but all that I accomplish is either
"bunching up" or "stretching" my equation.
How do I get it back to what it looked like originally?