When talking about food, to me, the word "portion" refers to the size of a serving, so to say "We should be eating smaller portion sizes" is redundant. It should be, "We should be eating smaller portions."
Although, I suppose you could say something like, "My portion has has more onion." In this case, you clearly don't mean "The size of my serve has more onion," and the word 'portion' is referring to your serve in general, rather than just the size of it.
That said, the phrase "portion size" still doesn't seem right to me. Is this tautology, or something a bit more subtle?