Maybe this will help, even though I never tried it myself because I like opening large and hairy files :)
Type the following: C-h v large-<TAB>
Then the description for the variable will show up like this:
large-file-warning-threshold is a variable defined in `files.el'.
Its value is 10000000
Maximum size of file above which a confirmation is requested.
When nil, never request confirmation.
You can customize this variable.
This variable was introduced, or its default value was changed, in
version 22.1 of Emacs.