I just enter the name of the food and make the serving size part of the name. As in:
Klondike Bar one serving is 1 bar
Then I put in all the information for the food. If I eat 1 and a half Klondike bars, I log it as 1.5 servings and fitday does all the calculations without needing to know weight in grams, oz., etc.
If I put a food in like: Utz Potato chips 1 oz is 1 serving
then, if I eat 2 oz, I just put the number of servings as 2.