ExtraSize
This variable specifies the additional installation size, in bytes, over and beyond the total size of the installation files. You can use this variable if your custom actions create or install additional files (for example, by copying existing files or extracting files from an embedded archive) and want the installer to make sure that enough disk space is available. During installation, the installer adds the ExtraSize value to the size of the selected installation files and uses the total when it checks the available disk space.
Usage
Used internally by InstallMate. You can refer to this variable as <ExtraSize>. Set this variable to the additional installation size, or set to 0 (or leave empty) if your installer does not need additional disk space.