I believe if you first create the food (at least name it), then it'll take you to the next screen wherein you can change the amount. What I've been doing is creating a shell (with a name), changing the amount in the applicable serving (indicating g, kg, oz, whatever), then going back and editing the food, entering the actual nutritional numbers at that point. These are extra steps that didn't used to be required, but it's the best way I've found to address this now.
What I don't understand is why you can't adjust the serving size when you initially create the food, and why you are apparently no longer allowed to use cups or other volume measurements. I've had a FitDay account for several years, but haven't used it in a while. I used to be able to enter any food and adjust for any serving size I wanted (cup, tbsp, etc.). The fact that I can't do this easily anymore is very frustrating.
Especially since I can go back and edit my old custom foods based on volume. Can someone from FD explain why this functionality was removed?