In FieldMaker there is a property associated with the number of lines to display in the combo-box drop down. This property is inner lines.  It defaults to 5.  In this case, you want to change the value to another number of your choice.

To do this:
  1. Go to Tools > System Utilities > FieldMaker Configuration
  2. Click on the Layout tab
  3. Remove the field from the Layout
  4. Save the Layout
  5. Go to the Field tab
  6. Select the field from the list  
  7. In the View-As column select the combo-box and click the properties box at the bottom
  8. You should see the inner line property
  9. Change the value to the desired number of inner display values
  10. Return to the Layout tab
  11. Add the field back to the appropriate location
  12. Save the changes on the Layout
  13. Commit your FieldMaker changes