We have never done the dining plan (haven't been back to Disney since it started), but we have used the old Vacation Kingdom Lunch vouchers and shared our meals. It was me, DH and two preteens. We are all pretty light eaters.
My opinions on some of your places:
Alfredos:
We got the pizza appetizer (dinner plate size) and chicken parmesan. I shared this with one DD. We also had a chocolate mousse dessert. This was more than we could eat and left some of the chicken and one piece of pizza.
DH and DD shared the pizza appetizer and the pasta trio. DD ate the pizza and DH had the trio. They got a cheesecake mousse type dessert.
Mama melrose.
We did the fantasmic package and thus had 4 full meals. NEVER AGAIN!! We brought home (back to the hotel) 2 full meals.
The chicken parmesan meal was huge. It had 2 full breast pieces and a pile of spaghetti. Me and both DD could have shared one! (and had some left). We also had the salad and flatbread (pizza) appetizer. DH got a pork meal with polenta (?? I think that's what it was.) He didn't share. It may or may not have been big enough to share depending on your appetite and what appetizers and dessert you ordered. For dessert we had a tiramisu (DH ate this), cheesecake (me), ice cream (DD) and I think chocolate cake thing (DD).
Chicken parmesan. The pasta is underneath and you can't really see it.
Sci Fi:
We have shared here for lunch, but the meal was smaller. Still enough for us. DH ate the chili appetizer. We spit the flatgrill steak sandwich with onion rings. They split it in the kitchen and brought us our own plate. We didn't get dessert and were full enough.
This is one half of the steak sandwich and onion rings.
I can't recall if Whispering Canyon Cafe was on your list, but as long as you do not order the All YOu can Eat Skillet, you can share. DD and I shared the roast chicken meal and it was PLENTY to share. DH got the skillet and didn't share. But he didn't ask for seconds of anything except another piece of cornbread. My other DD ordered the kids PB&J meal.
I hope it helps to see some pictures to get an idea about portion sizes. I also recommend looking at the "Post your pictures of Disney Food here" thread and seeing what restaurants look "sharable" and which do not.
Maggie