Thanks a lot for your prompt reply, David.
1. Can I regard sentences 1-3 interchangeable and 4-6 the same?
2. I was thinking 'sentence 2' could be wrong because 'size' is just one of the characteristics of the products. Writing "The two products are the same size" seems to put an equal sign between "the two products" and "the same size", which sounds wrong to me. I hope I did express myself clearly and please comment.
3. If I change the word 'size / sizes' to 'quality / qualities', are they still correct?
Thanks a lot.