Several years too late:
Just for completeness I want to give yet another answer:
First, go to Excel-Options -> Formulas and enable R1C1 references. Then use
=CELL("width", RC)
RC
always refers the current Row, current Column, i.e. “this cell”.
Rick Teachey’s solution is basically a tweak to make the same possible in A1 reference style (see also GSerg’s comment to Joey’s answer and note his comment to Patrick McDonald’s answer).
Cheers
🙂