It is possible that all there is not enough room on your line to grow the field. You can try a few things :
- Make the form width wider by dragging the right edge of the form or via the Form Setup dialog (File > Form Setup... and change the Form Width)
- You can make other field on the row narrower (if they are not already at their minimum width).
Each field type has a minimum width. What field type are you trying to make smaller?
For example : Text and Multiline can go down to a width of 30. Dropdown min width depends on the length of the long item in the dropdown list.
Hope this helps