1. Can I regard sentences 1-3 interchangeable and 4-6 the same?
Hello again, Taiman,
Sentence (5) is not interchangeable with (4) or (6). Moreover, (5) is the weirdest sentence in the batch. Normally, we would use the verb "come" there: "The two products come in different sizes." The sentence means that each product has a variety of sizes.
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.
Sentence (2) means that the size of each is the same. If it helps you, you can think of "of" as elided in (2). It is a very natural sentence and not to be regarded as incorrect. If you're inclined to consider it incorrect, you should consider that you haven't discovered the proper analysis.
taiman posted:3. If I change the word 'size / sizes' to 'quality / qualities', are they still correct?
Why would you want to make that change?