Normally, we would use the verb "come" there: "The two products come in different sizes."
That's a nice change, David. I've been also thinking of this other choice:
- The two products are available in different sizes.
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?
I guess Taiman might want to know if we can grammatically use the word "quality/qualities" in the same sentences (with an evident change of meaning). I think you will agree the answer is affirmative.
1'. The two products are of the same quality.
2'. The two products are the same quality. (This is the only one that sounds a bit weird to me.)
3'. The two products have the same quality.
4'. The two products are of different qualities.
5'. The two products come in different qualities.
6'. The two products have different qualities. (This is ambiguous, as "qualities" may refer to their degree of excellence or to their properties or features.)