The maximum number of characters allowed is 1,000,000
If your post is longer than that, you should receive a message that your post is too long and you should shorten it. You should not get a "page cannot load" error due to this.
I don't know what the problem could be, had a similar problem when trying to edit certain thread titles (only 2 times of all my edits so far).
Perhaps there are special characters that are not accepted? You could try this with a different title and in a different forum. You can then leave me a message to which forum and with what thread title you would like the post should be moved and edited.