Can we change the length of 'Opportunity Name' field in salesforce ?

Hello,

We cannot change the Character Limit for Standard fields however there is a workaround to restrict the limit through Validation rule.

Workaround for above Example in Salesforce Classic:1) Click on Setup | Build | Customize  | Accounts | Validation Rules.

a) Click New and enter Rule Name and Description.
b) Enter the Condition formula: LEN(Name) > 80
c) Enter Error Message - Account name should not exceed 80 characters.
d) Click on Save

In the Lightning Experience:

2) Click on Setup | Object Manager | Accounts | Validation Rules.
a) Click New and enter Rule Name and Description.
b) Enter the Condition formula: LEN(Name) > 80
c) Enter Error Message - Account name should not exceed 80 characters.
d) Click on Save

Note:  The character limit on any other Standard field can be restricted by using the sameValidation Rule, only the formula would differ based on the character limit to be restricted and the field name. The Formula for any other field would be:

LEN(FieldName) > number of characters to be restricted.