Could somebody please tell me the typcal size of allen screw for connecting a split stay joint or attaching a replaceable dropout?

if you are asking because you are fabricating them yourself, I would say 5mm would be a good choice. The dropouts might be 4mm, I've never looked at one.

I've seen pics with 4 bolts that look like 4mm bolts, but also with 2 big ones (6 or 8). But size and number will matter a lot less then proper tensioning in this situation, being a shearing kind of stress the bokts are subjected to. Either one would work I guess.

Thanks for the responses. I did make the split connector myself so I guess I can choose the screw size. I have seen just one used. I think if there is a step interface between the surfaces shearing forces are pretty much eliminated since the overlap takes all the stress. The screw only holds the two surfaces together.
I'll probably go with two since there is enough room for that.

there's no harm in putting two screws in there anyway but good one on that stepped joint - I missed that.