Hello Gitesh,

First, in the SQL table CMS_Class, update the column ShowAsSystemTable to True for the row WHERE ClassName = 'ecommerce.paymentoption'

Then, in Site Manager -> Development -> System tables, Edit the system table 'Ecommerce - Payment option'. Select the field 'PaymentOptionPaymentGateUrl' and update the field Attribute size according to your needs. Finally, click 'Save field'.

Second, you will have to modify the CMSModules\Ecommerce\Pages\Tools\Configuration\PaymentOptions\PaymentOption_Edit.aspx file. At line 69, on the cms:CMSTextBox with ID txtGateUrl, update the MaxLength value to the attribe size you defined in the first step.

Voila ! :)