ok. I think i got it. This is because your field is not a text field. that option is available only for the text fields. For numbers you have 4 options
1) Increase the width OR
2) Change the font size dynamically as i mentione din the above post OR
3) Create a new formula field that converts your number to string using ToText function. now when you place it on the report and right click and go to properties, you'll find the 'Can Grow ' option OR
4) drag a label on the report. now drag and drop your number field on this label. now right click on the label and select 'Can Grow' option.
I hope atleast one of these wrks for you. :)
