At least three people in our household (namely me, my wife and our eldest son) wouldn’t mind shedding a few pounds (actually kg on our scale, since we’re in Israel). 

Obviously the first step is basic info: getting a benchmark figure for the calorie content of a typical portion of challah. When I looked up user-submitted calorie info posted on websites that seem to have automatically generated figures, I get ridiculous data ranging from 80 calories per serving to 320 calories per serving; not very helpful, especially since there’s no indication what the “serving size” is. 
 
Websites maintained by bread bakeries and Jewish bakeries seem to have more reliable calorie info. According to Bread Alone Bakery’s website, a 2-ounce slice of challah bread contains roughly 160 calories, with 27 to 30 grams of carbohydrates, 6 grams of protein, 260 mg of sodium and a bit of iron. 
 
Challah bread contains about the same number of calories and carbohydrates as white bread, but is higher in fat and protein. 
 
A charming blog called Between Carpools describes how to have your cake (i.e. challah roll) and eat it too (without excess calories) 
 
“Obviously, if I’m counting calories, I can stick to one full matzah and I’ll know exactly how many calories is in each one,” she writes. That’s a good strategy to stick to her diet plan. There’s just one little problem: “But I like challah. Like a lot. And I really really don’t want to give it up.” 
 
So she set out to create a challah roll “in the perfect kezayis portion.” (What’s a kezayis? Scroll down.) 
 
“The first problem with creating this challah roll,” she writes, “was that my usual weekly recipe had lots of eggs and a little more sugar than necessary. I’d need to start with a lighter version. Instead of experimenting, I reminded myself that my sister-in-law Devorah makes delicious challah with less eggs and sugar. Her recipe uses a bread machine, but I’ve easily made it in a mixer for when I need to make egg-free challah for some with egg allergies.” 
 
“Now that I had a lighter challah recipe that I knew was good, the next step in making my rolls was to figure out how much is the minimum challah one needs to eat in order to be able to wash and bentch. Ashkenazim and Sefardim have different minhagim, so ask your LOR. Based on Rabbi Bodner’s sefer, the two-ounce roll that I made seems to cover everyone.” (According to the Rabbi Heber, cited below, it seems you’d need 2.6 ounces.) 
 
You can find her lighter challah recipe here. 
 
I have a feeling a lot of readers don’t necessarily have the initiative or available time as the creative Between Carpools mommy. So the question then becomes what chunk of challah should you set aside during the Shabbos meals. 
 
Rabbi Dovid Heber of Star-K explains that a kezayis is 1.27 fl. oz. (38 ml) and writes that in the case of bread/challah it should be the size of a golf ball. What Ms. Between Carpools didn’t mention is that really you should eat a kebeitza during each of the Shabbos meals (i.e. two golf balls). A kebeitza is twice as much as a kezayis. In some circumstances, health concerns may be an overriding factor, and your rabbi may advise you to suffice with a kezayis. 

One aspect of challah calorie counting that nobody seems to mention is that often more calories get smeared on your challah. A bit of hummous or eggplant spread. So limiting the platform used for those spreads is going to further curtail your Shabbos calorie intake.