On the Energy Shield Generators it says it can shield a small, medium, large and enormous factories.. Is that number of objects, area, or what metric differentiates them ?

Area, internal battery recharge speed and it's capacity

Could this information perhaps be put as a description on the item and recipe to make it, along with the exact numbers?

You already see internal buffer capacity (in tooltip) and max power input on shield (both when you place it and when you look at tooltip). Area is defined by runtime config values and hence can not be present in description.