To determine the most likely size, compare the recipe to what you find in the produce aisle of your local grocery store. Chances are, the loose potatoes sold in bulk are around what we often refer to as a 70 count, i.e. 70 potatoes in a fifty pound carton. Now that term makes sense in foodservice, but not to consumers at home. Typically a 70 count potato averages 11-12 ounces.

For a small potato, I use the USDA standard portion size, 5.3 ounces. If the size is crucial to how the recipe turns out, then the recipe writer will usually call out the measurement in ounces rather than just saying “each”. 

By the way, that small potato is about 110 calories each.