File size exceeds configured limit (2560000), code insight features not available

In IntelliJ 2016 and newer you can change this setting from the Help menu, Edit Custom Properties (as commented by @eggplantbr).

On older versions, there's no GUI to do it. But you can change it if you edit the IntelliJ IDEA Platform Properties file:

#---------------------------------------------------------------------
# Maximum file size (kilobytes) IDE should provide code assistance for.
# The larger file is the slower its editor works and higher overall system memory requirements are
# if code assistance is enabled. Remove this property or set to very large number if you need
# code assistance for any files available regardless their size.
#---------------------------------------------------------------------
idea.max.intellisense.filesize=2500

This is built from Álvaro González's answer and How to increase IDE memory limit in IntelliJ IDEA on Mac?

Go to Help > Edit Custom Properties

Add:

idea.max.intellisense.filesize=999999

Restart the IDE.

Edit config file for IDEA: IDEA_HOME/bin/idea.properties

# Maximum file size (kilobytes) IDE should provide code assistance for.
idea.max.intellisense.filesize=60000

# Maximum file size (kilobytes) IDE is able to open.
idea.max.content.load.filesize=60000

Save and restart IDEA