So if you can deliver them (and you think of them) they are easy to make? How much would each one weigh?
Easy is a relative term. They are no more difficult to build than a conventional bomb, which is not difficult. The triggering system is a bit more difficult, but most mines pre-WW I were contact detonation, so as long as the exploder is designed to survive the initial deployment you are good to go.
Weight is a variable. The goal you have is to be able to sink a pre-dread BB, so minimum size would be around 250 pounds of wet gun cotton as a filler plus the casing. Figure 500 pounds for a drift mine and 1,500 for a moored mine. Drift mines are exactly what they sound like, you drop them and they bob along with the current, very nasty due to their unpredictable nature, but just as dangerous for your shipping as the enemy's. A moored mine is a more predictable weapon, at least you know where it is within some degree of certainty (anything you drop out of a plane is going to have a degree of uncertainty attached).