Description

How to change the length of data which can be stored within a label 

Procedure

The label information is stored in three different columns in the sys_dictionary table.

table: sys_db_object 
column name: label 

table: sys_dictionary 
column name: column_label 

table: sys_documentation 
column name: label 

Each of sys_dictionary records listed below will need the "max length" field to be amended to your desired size : 

Steps

1. Navigate to System definition > Dictionary
2. Open the filter and add the following search criteria

table is one of sys_dictionary, sys_db_object, sys_documentation

AND

column name is one of label, column_label

i.e.

https://<INSTANCE NAME>.service-now.com/sys_dictionary_list.do?sysparm_query=elementINlabel%2Ccolumn_label%5EnameINsys_db_object%2Csys_dictionary%2Csys_documentation

*Replace the entry <INSTANCE NAME> with the name of your instance

3. Open each of these records and update the field labeled "Max Length" to your desired size

Additional Information

Increasing the label length is not recommended. These fields are core out of the box elements and amending them will create customer updates which will prevent future changes being applied to these fields via an upgrade. 
Labels are designed to be short, there should never be a need to have more than 80 characters in a label.