Through the UI JIRA has a maximum limit of 254 characters for single line text custom fields. Using the Jira Suite Utilities post-function 'Update Custom Field' I was able to set a single line text field to 255 characters. Trying to subsequently edit the same 255-character-long field value through the UI yielded slightly different behavior for different browsers. In Chrome you could delete the final character but not restore it (as the 254 character limit was still imposed). In Firefox you could not delete the final character until you had deleted any other character to reduce the field size below 255.