3/4 or 5/8 s would probly be overkill for a dropin. I would go with 1/2 unless your supports are really close together and you made cheap. Then you could use 1/4. really depends what you are using it for. If if needs to bend alot or if its for strenth, and how big of a span it has to cover. 5/8 or 3/4 wont really bend but it is strong enough to span a pretty big gap.