You gave the question about photon size and it seems did not satisfy with an answer you got.
And you did not satisfy with others answers given by mentors.
Let's come back a bit starting with your original. Why do not find the answers youself?
As Descartes said, the complex question should be split to the several simple questions in order to be resolved. Now we call this baby steps.
What do you think about the following, in some sense less difficult, but more constructive, questions:
1. What does mean 'to measure the size' in SR and in QM?
2. Can we measure 'anything' in Nature without direct or indirect implementation of the electromagnetic field?
3. What is the 'size of the electron' and how to calculate it in SR and in QM?
4. How to measure the electron 'size' and what would be the results?
5. Why are we not asking about the geometrical size of plane electromagnetic field in the 'classical' physics (SR)?
6. How is the notion of the 'classical' electromagnetic field transformed to the notion of 'the wave function' of electromagnetic field in quantum theory? In other words, how is the photon appeared in quantum theory?
7. Is that any difference in the interpretation of 'the wave function' of an electron and 'the wave function' of a photon in QED? If so why is it?
If you find the answers yourself digging the books, solving the problems, etc., you will feel more comfortable with the answer you receive from ZapperZ and others to your question 'How big is a photon?'
If you don't, I am sorry to say, but the advice 'go and study QED' should be extended to SR and QM. Nobody can help you with that.
It's difficult, but there is no other way if you want to understand modern physics.