How do I set the display size for custom fields?
The Add/Edit custom field doesn't provide facility to add additional attributes but the Custom field table does have a column to store this attributes and this field is used while rendering the custom field. You can do this by any one of the method below


Create a extension and use buildform() hook to add the attribute field on Add/Edit Custom field form. Once its added on the form after submit it will be automatically processed and stored in db by Civi.
Once you have created the custom field you can use API explorer(CiviCRM >> Support >> Developer >> API Explorer) to update the attribute for a custom field as shown in below screenshot 





  Note: Make sure you add space at the end for each attribute as this is concatenated with other attributes while rendering the field.


Thanks

Pradeep