When in doubt, and really stuck or in unfamiliar territory -- I'll go with the children's sizes. If I have a chance, I look it up online really quick.
In general, I prefer Subway, Firehouse, Panera type "sandwich" places to "burger" places or popping into a grocery for sushi to go. More chances for some fresher veg options.
If I'm stuck with McD's, then I decide if I want to indulge in fries or not. If yes, I'll get a plain small fries and cheeseburger. If no, then I'll go with the grilled chicken sandwich and a side salad with the LF balsamic. Drink water. I try to keep the whole thing at 500 ish calories.
It's nice that they have the exchanges online...
http://nutrition.mcdonalds.com/nutri...exchanges.html
Here's other fast food tips.
http://helpguide.org/life/fast_food_nutrition.htm
For me the calories is something I usually can manage but its the sodium that kills me. Yargh.
HTH!
A.

It's yummy and fills me up.