Features are automatically turned on and off depending on the size of the document as follows:
- XML Documents < 2 MB - All features (Syntax Highlighting, Document Outlining, Intellisense) default value = ON.
- XML Documents > 2 MB - Document Outlining default value = OFF.
- XML Documents > 15 MB - Large File Editor is used by default*.
*You can change this value using Tools->Options->Environment->Max File Size in MB. Values of 30MB+ have been used without issue, but performance degrades with file size so 15MB is the default.
The Large File Editor can open files of any size (tested to 2GB+) as it pages the data from disk. This restricts the functionality, but still allows you to use the tools Format Document and Validation.