Melissa,
There are limited things you can do. Because a checkbox is square - in order to increase the checkbox size, the height of the row may also have to increase.
To increase the size of the checkbox in the Object palette>Cell Tab>Size (default is 10pt). In my example screenshot - the large checkbox is 40pt. If the column width remains significantly wider than the checkbox width, the box will be right aligned (no caption) or left aligned (with caption). The user will have to click the actual check box to select it.
The only other option is to place a button in the same table cell. Make the button have no color, and no caption. Then set the click action to select or deselect the checkbox. Note: this is not recommended if your form needs to be accessible to persons with disabilities. The visual appearance and the functionality don't match, so use with caution.
If business requirements allow the form to be redesigned a little, have you considered an alternate layout to the multiple pass/fail columns? Say a drop-down list instead?
Screenshot below, and sample PDF attached.
Hope this helps some.
Angie