In this case, it's specific against SSL. But in general, this is another form of differential crypt-analysis. Any credible encryption system takes steps to prevent this. (simply put, a single bit change in either key or plaintext should not have an easily predictable effect on the ciphertext.) As far as I know, no one has tried this method on other crypto methods.
Size alone is a very weak means of mapping content. Almost every modern web application has some variability in the output size at any given URL. Plus it's likely there will be many URLs generating the same size output.