My DD (13) and I share often, but we are not big eaters.
At alfredos, the pizza appetizer was a dinner plate size pizza. The chicken parmesan had a huge piece of chicken and some pasta. The dessert was a dessert cup of chocolate mousse. We had more than enough to share. My other DD could have probably shared with us too, but she got the kids spaghetti.
At Whispering Canyon Cafe, DD and I shared the roasted chicken. It was a lot of chicken, I think maybe 1/2 a chicken? It was more than the two of us could share. We didn't get an appetizer for that meal.
DD and I shared at Teppanyaki. It was plenty for us. They gave us each a plate of the veggies and rice and then just split up the meat. I ended up eating all of DDs meat, but only because it "tasted funny" to her. Otherwise, it would have been enough for us to share.
At Tempura Kiku, DD got the shrimp appetizer ( I think it was 3 of those large shrimp / prawn tempura). It was enough for her. I got the fruit appetizer and then picked a little at my other DDs kids meal when she was done.
At Mama Melrose, we got the Fantasmic Package one time and we brought home 2 full meals! Way too much food for us. My chicken parmesan was enough to feed me, DD13, DD9!! It was 2 huge breasts with pasta. Plus we had the salad and the flatbread (pizza) and the dessert!!
So, these are a few places that I have personally experienced that had portions enough to share. Check out the "Post a picture of your Disney food" thread that is stickey-ed at the top of the Restaurant board. You can get a good idea of portion sizes and go to the places that look like it would be enough to share.
We are planning on getting the dining plan this June. Both of my girls will be "adults" at this time. I already know that getting a full meal for each of us is WAYY to much food. So I am planning on splitting a few meals (we will get 2 full meals at lunch and 2 full meals at dinner and share). We will use the counter service for breakfast. We will have the snack while watching fireworks/evening shows.
Maggie