magento 2 - modify db_schema.xml field length

You change that Field length from Custom module.

Create db_schema.xml at your module and add below code:

<?xml version="1.0"?>
<schema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:Setup/Declaration/Schema/etc/schema.xsd">
    <table name="catalogrule_product" resource="default" >
<column xsi:type="varchar" name="action_operator" nullable="true" length="50" default="to_fixed"
                comment="Action Operator"/>
    </table>
</schema>

And your module should depend on Magento_CatalogRule

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="{VendorName_ModuleName}">
        <sequence>
            <module name="Magento_CatalogRule"/>
        </sequence>
    </module>
</config>