A lot of places have menus online with nutrition information provided. If you know where you're going, look it up and plan what you're going to have, and stick to it. If there is no such information, ask the server how the food is prepared.
I have a stack of such menus that I've printed out.
You'll also have to make some decisions beforehand, too, such as order an appetizer and skip dessert, or vice versa, as well as if you're going to drink alcohol.
As far as bigger portions, the easy way to handle that is ask for a box. Eat your normal portion and take the rest home.