I went to the Dixie site, and they give no clue about screw size in the part description.
They only list one rear sight for the 1858 which requires screws, which is the adjustable target sight, part number EP 2600.
If this is the sight you have, just call Dixie's Information line (1-731-885-0700) and they should be able to tell you which drill and tap you need.
Or, if you know someone who is a machinest or gunsmith, they will have a thread gauge you can run it into. It will probably have metric threads.