Hi Poppins3,
Here is a website link that gives many different conversions for cantaloupe. (remove the space from between the "www." and the "the" to get there)
http://www. thecaloriecounter.com/Foods/900/9181/Food.aspx
I would just pick the one that best matches how you weigh or measure yours and go with that and create a new custom food or edit the FitDay. I use the FitDay PC and love it. Hope that helps!
The closest match I found was one small wedge is about 55 grams, equal to 19 calories. So if you have a scale that would be your best bet to get an idea of how much that is. A cup of diced is much more. Diced being about half inch cubes, you would fill a cup and get about 156 grams. About three times as much and about 60 calories.