I'd try a macro from Debra Dalgleish's site:
http://contextures.com/xlcomments03.html#Resize

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm

Trish wrote:

Hi, I am using the Comments facility in Excel 2003 - often when I type in a
comment, then click out of it, I find that half of the wording is hidden and
it necessitates me going back into it to resize it. I think there is an
automatic sizing feaure somewhere (??), but the time it occurs the most is
when I edit the comments and add more text - the box does not automatically
resize and that would be very helpful. In Excel 2003 I can double click at
the top of a column and it will set the column width to the widest row in
that column. I wondered if there was a similar feature for sizing and
resizing comments boxes. Many thanks

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Dave Peterson