As the size of sweet potatoes varies greatly, along with one's opinion, just what would be the weight of 5 medium sweet potatoes?

Victor, same possible approaches as with the eggplant dish yesterday.
1) Cooking is not baking, which is more like chemistry and requires precision. If the recipe author is casual with sizes (I admit, sometimes it is maddening), you can be equally casual in the making. Trust yourself as a cook, go by eye, keep notes. If you like the results, repeat. If you liked the recipe, but want more or less of that ingredient, adjust next time you make it.
2) Use various sources, like the links Lindsay-Jean and I provided yesterday. Or do your own web search...USDA, various nutritionists, dieticians, etc keep lists of ingredients, sizes, their constituent nutrients.
Good luck with the soup...it looks tasty. And happy Boxing Day (from the Commonwealth) or post-Christmas (in USA) and NY (everywhere)...:)