ahoy @Lithesh,
the max length of each particular field is set on its system dictionary record.
You can either go to sys_dictionary table, locate your table, the field and update it. Keep in mind that some field might be inherited (e.g. field visible on Incident form may be defined on Task level)
1) Go to [sys_dictionary] by navigating to
https://yourinstance.service-now.com/sys_dictionary_list.do
then selecting your table/field, add "Max length" column as it is not present by default
2) Another option of doing the same is to click right-mouse button on that particular field and select Configure Dictionary:
It will get you to the particular record under the sys_dictionary table for that particular field.
Then you can update the desired lentgh per field. There's no difference in editing in list view or directly this form, updating the very same value.
3) Going to a table definition allows you to update it
On a record, go to triple dots > Configure > Table and it gets you here where you can also update the max character lengtht
All the three approaches do the same, it's up to your preferences what works the best for you or depending on the nature of what you want to achieve:
1) is good for mass updates of more fields at once,
2) is the fastest method for a single field (as if it is inherited it will get you to the parent configuration, it is possible to do override - Task 40 characters and Incident 120 characters)
3) is good for investigations or while setting complex changes
EDIT: re-uploaded attachments as they were saved blurred
