No --

a) you don't "approach the end of the cell" when typing in Word. The cell
accommodates whatever you type into it.

b) there is no way to monitor keystrokes as events.

No.

There is no interruptable event which occurs whilst typing and no mechanism
to do this.

A cell can contain a practically indefinite amount of text. Although you can
constrain the visible size of the cell, the contents can fall outside those
boundaries. Normal typing remains at exactly the same distance from the end
of the cell all the time; no matter how much you type you will always be the
same distance from the end. So your real question may be something else.

Perhaps you have a Form Field within your table cell. If so you can restrict
the size of that but you still won't be able to get a warning about it being
nearly full.

--
Enjoy,
Tony