Usethe new "Export As" instead.
Save For Web is being phased out. It builds on the old Image Ready code from Photoshop 5.0 - 7.0 - ten to fifteen years ago, when 2000 pixels was considered a large file...
Where do I find "Export As"? (cs5)
Well, that's what you get for not staying current. It was introduced sometime later.
There's no way to disable the warning in SFW. It really isn't designed to handle big files.