I have a custom setting field of Text type. I ran out of 255 characters which salesforce provides. How to add multi line text ?
I am afraid but you can't create field with more then 255 character(in multi line) in Custom setting.
Same case with custom Metadata.
I think the only option you have for now is use custom object and manage your data there and if you are sure about data size then you can create multiple fields.
And There is also an idea on Idea Exchange and you can also Vote the idea.