Variation on the ECB Penguin problem
You cannot, at least not if that's the only encrypted image. ECB leaks information because repeated plaintext blocks lead to repeated ciphertext blocks. But it doesn't leak the information within those blocks directly.

So you could find the original size by finding another encrypted picture with the same header of which you already know the size. In that case the encrypted header should be identical.

Otherwise, smart guessing might be the best way forward. If you look at the vertical lines then there should be a high amount of repetition between one horizontal line and the next. Just shifting the image and selecting the one with the most vertical repetition should do the trick.