the Description Attribute has a hard coded limit of 762 characters
As an alternative, you can leverage the Objective attribute, which has an ootb maximum size of 2000:
As an even more alternate option, you could create a custom attribute under the Idea object. Now if you do this using data type 'string', you'll still have a 2000 character limit. However if you use 'Large String - Rich text' it will have no such limit:
Important: in order to make this available in the New UX, you MUST assign an API attribute ID to the attribute.
Once you create the new attribute in Classic PPM under the Idea object, you'll need to go to Blueprints in the MUX (Administration-Blueprints) Clear any filters and select the type 'Idea' and edit the default blueprint to add this field (and optionally remove the current Description field).