On the New Custom Food entry page, click on the words Custom Size, which you'll see just to the right of where you enter the serving size. That will allow you to work with amounts other than 100 grams.

To correct any Custom Food entries you have already made, click on your Custom Food, then click the box marked "Customize." You'll see that box if you look straight below the bolded word "Serving" on a Custom Food entry you previously made. When you have corrected your entry, scroll all the way down to the bottom of the page and click "Replace Custom Food." If you prefer, of course, you can click the other box, "Add as new Custom Item." That will add your corrected entry as a second listing of the Custom Food. The food will be shown twice on your list.

HTH!