Labels of custom fields can accomodate up to 40 characters. However, users can can type in more than the 40 character limit of Transaction Body Field labels when it is displayed in the Review and Submit Page of Web store:
1. Navigate to List > Web site > Tabs
2. Edit the Checkout tab
3. Click the HTML Source Code link in the Place Order Page Message field
4. Enter the example script:
5. Click on Save
Note: custbody15_fs_lbl is the ID of the transaction body field label in checkout page and the text on the right side of the equation is the replacement label.