The portions are big to me but I don't know what your appetites are like. There is no charge for sharing food at any Disney World restaurant except Shula's.
If you want to share something, just tell your server and more than likely, it will be split in the kitchen. I will warn you that you are probably going to get half the entree but an almost full serving of whatever side is included. So if you go the steak at 50s Prime Time, you'd get half the steak and almost the full serving of the scalloped potatoes and also of the green beans.