When you create a Roll-Up Summary field based on a picklist with multiple values in Filter criteria, you cannot use more than 255 characters.
For example:
To work around the 255-character filter limit, you can create a formula checkbox field on the child object that evaluates the picklist values using a CASE function, then use that checkbox field as the filter criterion in the Roll-Up Summary instead.
1. Create a checkbox (formula) field on a detailed child object and use the same in the Roll-up Summary filter criteria.
CASE( Country__c, 'Afghanistan', 1, 'Albania', 1, 'Algeria', 1, 'Andorra', 1, 'Angola', 1, 'Antigua and Barbuda', 1, 'Argentina', 1,0 ) = 1
f) Click on Check Syntax button to validate the formula and click on the Next button.
g) Select the appropriate Profiles for field-level security and click on the Next button.
h) You can hide this field from the page layout by deselecting the checkboxes, otherwise click on the Save button.
https://help.salesforce.com/s/articleView?id=platform.customize_formulas.htm&language=en_US&type=5
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