I don't know how eval works really, I've never used it myself, but couldn't you just do this:
document.eval('"ment" + i').style.width = 100;
I THINK the problem is that it's getting confused as to which variable to use, the variable outside the function, or the one inside.
What you could also do is remove the:
because if you don't use the var keyword in your functions to declare variables they are created as global variables.