tinymce-docs-4x/_includes/configuration/indentation.md

indentation

This option allows specification of the indentation level for indent/outdent buttons in the UI.

This defaults to 30px but can be any value.

Type: String

Default Value: 30px

Example:

tinymce.init({
  selector: "textarea",  // change this value according to your html
  indentation : '20pt'
});