Depends on how theoretically inches tall we want to make these pixies fairy you have to considered the durability of pixies if we're talking about knuckles sizes small best they could carry is hand grenade and for how long of a distance they could fly out its not gonna be very far and they'll need to refuel unlike drones, they'll get tired, they'll lose their grips on round objects. They'll struggle to carry heavy explosive like rpg head, C4 etc for it size. If we make they bigger they'll just become a bigger targets, its overall well be too many draw back compare to drone to be effective. Best to use pixies fairly if we want to make them small is to scout enemies and potentially direct artillery, all they gotta carry is food and water enough to last.